@khaipan You are lucky to be able to take Max to work with you. That will allow you to make a home alone transition when you are all ready. He will be sensitive to your feelings, so you need to be ready as much as he is. As long as you don't make a big deal about it, he will be less likely to get excessively attached. My guys are quite attached to me (and, if they are awake, will stay at the window, watching me leave even if my S.O. is here), but they are fine when I'm not here. I've not really acknowledged them when I've left, so they can think that I'm just out on one of many short errands. I can frequently 'sneak out' without them even getting up. Not a big deal.

Before trying daycare again, you might want to approach that at least as slowly as leaving him alone if he's not had the routine of getting together with other dogs. It might also be good to spend the first couple of 'retry' visits with him (hopefully the facility allows for this), letting him have a little fun before leaving. If he's not into other dogs, he will probably be happier home alone. Even if you don't plan on leaving him at daycare regularly, it doesn't hurt to help him learn to like a place just in case you have an emergency need for him to stay.

As mentioned, consulting with a behaviorist is a good idea. Before allowing anyone to work with your dogs, watch them work with other dogs to see if their style is a good fit for your family.
I would also make sure both are checked thoroughly by a vet, bloodwork (including thyroid testing) included, to make sure that health issues aren't making one or both uncomfortable. I know of times when a basenji sensed a terminal condition before it even showed in bloodwork. Are they both spayed? If not, going into season may be affecting either one's behavior.
While Mia heals and you look for the right behaviorist, keep them separated. This may require two layers of fencing to make sure that they don't start fighting through the fence. I've had over 200 basenji fosters and have kept newbies behind a layer or two of fencing while they learn how to behave here. Particularly when the more challenging ones arrive, nothing is for free, including access to human furniture. They get 'their spot' in the house (in their pen where they can still see the others) where they can feel safe and not worry about others bothering them. I build up communication and confidence by starting (or going back to) basic commands with every meal (with food sitting at their feet): sit, stay, leave it (a VERY important command), look at me, eat. Go for walks together, each human holding one, keeping them separate, yet close enough to be aware of the other. The outside world can be a good distraction to keep them from focusing on hating the other. When I introduce (or reintroduce) them to other dogs in a securely fenced yard, I start at times that aren't so exciting - quiet times, nap times, etc., not around feeding time or when we have visitors. When they are relaxed, you can work on trying them in the house with both of you there, each holding one on a leash. It is important to not make a big deal about it - just sit in a room together, using the leash to remove one to her safe area if she shows any apprehension or aggression. Go very slow. Don't rush it. That is when failures happen.
I hope Mia heals quickly.

@ripthebasenji, bloody discharge will come from the vulva, which will also appear swollen. No questions are dumb!
Hope you can get her into your vet quickly.

I have fostered many basenjis, ~200, from many different backgrounds and have b-sat intact males and females. I regularly have more than ten basenjis running together. Some seem to prefer pals of the same sex. I've had to be more careful with some males together, but the same applies to females. The ones more challenging were larger, so my experience has been that more males were a little more difficult to integrate than females since the males tend to be larger. Perhaps their slightly larger size made it easier for them to intimidate prior owners and set up a behavior pattern that was difficult to change.
To minimize the potential for issues, they are not left together unsupervised until it is quite apparent that they know our rules - no one gets to be top dog. No fighting allowed. Even when they are playing, I end the fun before it gets out of hand. I have seen many times when a couple started to get too rough. They do not forget quickly, so I try to make sure no one has a bad experience particularly prior to having lots of good interactions.
Note that all of this may go out the window if one gets sick. I have had fosters who were fine in adoptive homes for a year, when suddenly attacks started. As one example, the attackee ended up having lymphoma and had passed within half a year of the first attack. One really just has to constantly be vigilant because the game can change at any moment. That's part of the fun with basenjis.

Whenever we leave, we keep the tv or music going so that they never really feel like we left. They are so used to us coming and going for shorter spells or just going outside to take care of outside chores that they don't get worked up when we go. Of course, we do have quite a few in the house, so they do have company. Even those that have had separation issues in other homes where they have had companions do well here, though, perhaps because we are in and out so frequently. Since you only have a day to come up with a solution, you won't be able to 'practice' the frequent going and coming that we do.
For some, the tv playing sitcom shows may make them feel like family is still 'living around them' even when they are left alone. A quick solution may be doggy daycare until you have more time to do the frequent going and coming practice. You can also set up to have a two-way communication with him instead of just a camera check. I've used Skype and FaceTime to do this. Just hearing our casual talking on FT seems to make a difference for some who didn't stir for hours until the FT audio was muted. There are also remote controlled treat dispensers so they can really think you are magic and always with them even when they can't smell you.
Hope you and Max find a solution that works!

I've found a FaceBook group with files (with references) that are very helpful. I don't think you have to join to go through all of the files. They may also offer more advice if you join and post, but won't necessarily provide a specific diet: https://www.facebook.com/groups/211455130573/
I've had around a dozen Fanconi b's spend time in my home. Currently, I have an 11.5 year old Fanconi b boy who has been at stage 3 CKD for at least 4.5 years. He first came to me the day he was to be euthanized 5.5 years ago at 2/3 of his current 'healthy' weight. He was scheduled to be euthanized because of his poor condition. Just recently, he has tested at the top end of the range for stage 3 CKD, so I am just now modifying his diet. He has not been on a kidney diet for all of the years (5.5) that he has been with me. I did, however, always make sure that his meals were well-hydrated. He has been on a kibble diet with 26% protein - kibble always soaked in water prior to feeding (amount of water has been increased so that the relative moisture went from being like cooked food to raw food and now, with more water added, like renal canned food). His environment and handling has been much more important to his overall health than a kidney diet - he was extremely stressed and very untrusting of vets prior to his arrival here. He has a couple of other issues that are currently affecting his Fanconi (and kidney) condition.
While dealing with his issues and the kidney disease of other dogs, I've found that the knowledge and practice in the vetting community varies greatly and have seen varying consequences because of it. I've also found that getting the right supplies can be a challenge. It sounds like you are well-equipped to be your canine patient's advocate. Keep asking lots of questions. If you would like any more feedback from me, please let me know.

Perhaps a narrow ramp (with carpeting) on one side of the stairs might help.
I have seen improvement in a couple of seniors when higher doses of omega 3s and Vitamin E were added to their diets. MaxxiSAMe has also helped one with poor motor control.

Sorry to read he is having difficulties alone. I do not recommend the Impact crates, based on the experiences and conditions of one of my former fosters who was returned to me after the crate did not help in that home. He has a leg issue now. I'm not sure if it is a result of panicking in the crate. I have seen dogs damage, even bloody, themselves in crates, including soft plastic. I've also seen teeth damage and damage to some very expensive crates. There is one, a ZenCrate, that is designed to provide relief to those with separation anxiety, though the effect may be realized by leaving music on in the room with the crate. I have no experience with it. I did have one foster who was actually much calmer in his crate and very anxious (and destructive) if left outside of his crate. He was an exception...

Do you currently leave music or tv on? I find they do well when it is just loud enough to cover outside noises - not too loud. Depending on the dog's needs, I find different music or tv shows to be helpful. Happy sitcoms cover the silence with the sounds of people still in the house doing things. 'The Golden Girls' does that while also providing a soothing soundtrack that has put many of our basenjis to sleep. Soft rock has been shown to be soothing in canine studies.

How much in and out practice have you done? By that, I mean going in and out so many times starting with just a very short period of time that your boy won't know if you will be back in two seconds, two minutes, or ten minutes - starting out with just seconds, long enough to lock the door. I don't acknowledge anyone when coming or going when I'm doing this training until we are sitting down, relaxed, on my schedule. First times in and out should be so short that you are back and sitting down before he appears to react. This may mean that you pick up your keys, grab the doorknob, then go sit down. Lots of baby steps.

I prefer to have windows set so that they can watch squirrels, etc., out a back window, yet not see me going and coming out the front window. Blinds can be a safety hazard and face destruction. Our blinds on the front window are on the road side (which cuts down the solar heat load in the house while keeping the blinds out of reach), but you might need to fence off front blinds in your home. Sills may be destroyed.

Two way communication monitoring may help if there is enough noise at your work and during your commute for him to feel like he is still surrounded by life.

I've never not allowed a b to sleep with me unless he had to learn to be less defensive while sleeping first. If I can move them or have them move without being defensive, they are allowed on with everyone else. I don't make a big deal about it. Rather than keeping them out of the bed, I will block them from accompanying me to the bathroom, garage, kitchen, etc.

Since he has been good with just one one companion, adding a new one to the pack may do the trick. You could foster or 'borrow' a friend's dog for a couple of days first to see if this might work before committing to another companion. I do have one basenji who has not been happy when left home alone even if there is another b in the house. He is fine here, though, where there is a pack. One is not enough for him. He needs lots of company..

I hope you find the magic he needs!

@beth314 Here are a couple of things you might try:
Seek a vet who will do an outside visit. I have one local low cost clinic who does that for the difficult dogs.
Train your pup to wear an Elizabethan collar at home. I’ve used a very oversized one for one foster b with a previously very bad handling history (covid has not allowed me to do my usual rehab with him - going in to vet visits with dogs he knows without being handled - just a ride-along). It is so large that his nose does not stick out the end. In the inside of the cone, I spread peanut butter which he loved. Obviously, he could still bite, but the huge cone allowed him to be restrained gently from the side while examined. He growled a little, but did not attempt to bite even when he was stuck for his rabies vaccine. If she gets used to this at home, she can wear the oversized collar for the vet visit and feel less threatened than she might with a muzzle. Close-up checking of ears and eyes may have to wait, but at least vet visits might be a little more positive. An Adaptil collar may help to sooth her as well.
To ‘train’ for a vet visit, maybe you have a willing friend or family you can visit with her who can pretend to be a vet or vet tech. Go slow. Choose a kitchen or ‘sterile’ spot in the home that might be similar to a sterile exam room. At first, just have your friend ask her to sit and give her a treat reward. No touching until she seems comfortable. When she’s comfortable there, you perhaps could meet with family or friend at a pet store, going through the same slow get to know you process.
I hope you can find a vet with the right body language!

Thank you for helping this boy! Sounds like you are headed in the right direction with him.

Here are some tricks I've picked up from fostering a feral pack and around 200 basenjis from different backgrounds:

Webmaster harnesses (ruffwear.com) are very secure - a trim basenji with a definite waistline can't pop out of this one.

To get the feral ones used to collars and having collars and harnesses put on them, I introduced them at mealtime. I started out very slow - just draping a rope or leash over their necks when they were eating. They were very food-motivated, though there were a few who I had to start with even more slowly - just leaving a rope or leash next to their food dish while they ate. As they became more comfortable, I would put more on for each meal until they were finally wearing harnesses without a fuss.

To get them used to their nails being done, I used scissor-type clippers with a gentle curve. I let them stand with all four paws on the ground and clipped just the tip of one nail at a time without holding the paw. I would do just one or two at a time with a treat in one hand. They are so easily handled now - I can hold paws without any objection.

Many in the feral pack were too terrified to leave our property, even with other pals along to comfort them. For them, I carried them around for our first 'walks' off of the property, not making a big deal that they were in the safety of my arms - we were just going from point A to point B. After a time or two, they were not trying to hide their heads under my arm. I would then set them down on the ground at the halfway point to have them walk back, not making a big deal if they were mostly crouching.

I've never pressured them with visitors, letting them see I am (and the rest of the pack is) comfortable with visitors or strangers. While I didn't force visitors on them, I also didn't leave access to places like underneath furniture. They did have safe, out of the way spaces where they could go, just not places that left them completely invisible. When walking, I try to make a point of saying hello to people or dogs we see so that the fearful ones will see I'm not afraid of someone approaching. Those fearful of visitors or strangers eventually see by example (mine or the pack) that other humans or dogs aren't bad. Barking dogs were a bit more of a challenge for me to set a good example (I like our non-barkers!), so I tried to compensate by taking in and letting out deep breaths, yawning or talking - "there are those noisy guys who talk too much."

Handsome boy! I don't see any basenji, though. If you would like to do a DNA test for health that also tests for breeds, Embark, which is affiliated with Cornell University, is an option.
I hope you have a patient vet and that your boy learns to settle down and be cooperative there. I've had dozens of fosters who started out as real challenges for the vets, yet they all got over it and became good citizens with the vets. They don't necessarily love the visits, but they cooperate calmly.

I have little squeaky 'hearts' that I kept from destroyed toys that I can take out with me. The squeaky noise gets them to run to me (some are blow-through and can be very compact).

Have you tried real meat as treats? One of my formerly anorexic basenjis liked desiccated chicken (I never understood the appeal of 'cardboard' like meat to him). Is there anything special that he really likes (attention or toy, etc.) that might be considered 'a treat' to him? To reinforce the recall, a treat can be given every time the toy squeaker is used at home in a controlled environment so that it may still work when you are out at the park. Just save it for "really need the recall" times, though squeaking it when you are back at your car and can safely give him a treat would be a good idea.

@khaipan A few additional thoughts after yesterday's reply:

I've found that CBD oil is very dose-dependent and may require gradual increases to find an effective amount. I put it in the inside of lips when I give it. Some information I've seen online suggests that it is not as effective if given in treats or on food.

Former fosters have become much more anxious when given the usual sedatives. Benadryl seemed to provide a better response. One of those fosters was extremely anxious because one of her humans was ill. Max could be more anxious if one of his humans is ill - take care of yourselves!

What kind of mental stimulation are you providing? A routine that includes more obedience-type time (more frequent, very short sessions) can help to generally calm Max while building more confidence. Our guys go through a series of commands before every meal. Max may benefit from that communication more frequently.

I know of owners whose vets have not used Hemopet (or MiSU). Many were determined to have hypothyroidism by most other labs, frequently based on testing just the T4 level. The basenjis were prescribed twice the dose as those of similar size, thyroid function, etc., as was recommended on Hemopet reports. At least one of those basenjis showed signs of HYPERthyroidism when prescribed medication based on results from other labs. . All of the basenjis who were prescribed the higher dose, then given half the dose and retested 4-6 weeks later with a full Thyroid5 panel at Hemopet, had thyroid values well within range per Hemopet and any symptoms of hyperthyroidism disappeared. Hemopet and MiSU adjust for breed - sitehounds have a lower normal T4 range than other breeds. Both Hemopet and MiSU have more information regarding 'proper' thyroid testing on their websites. It's worth reading information from both sites, http://www.hemopet.org/hemolife-diagnostics/veterinary-thyroid-testing.html and https://animalhealth.msu.edu/sections/endocrinology/Thyroid_Canine.php

As mentioned, consulting with a behaviorist is a good idea. Before allowing anyone to work with your dogs, watch them work with other dogs to see if their style is a good fit for your family.
I would also make sure all are checked thoroughly by a vet, bloodwork (including thyroid testing) included, to make sure that health issues aren't making one or both uncomfortable. I know of times when a basenji sensed a terminal condition before it even showed in bloodwork. Is everyone spayed or neutered? If not, going into season may be affecting anyone's behavior.
Set up a safe spot for her that can be just for her. If she likes a crate, that can be acceptable. She might prefer a pen with a dog bed or something similar. This may require two layers of fencing to make sure that she doesn't start a fight through the fence. Go back to basics with training - nothing for free. Leave a leash on her in the house. If she growls while on the furniture with you, escort her calmly to her safe spot - "if you want to growl, you can do it here..." Her safe spot is not a punishment, it's just a designated area where she doesn't have to worry about sharing the space with any other canines.
I've had over 200 basenji fosters and have kept newbies behind a layer or two of fencing while they learn how to behave here. Particularly when the more challenging ones arrive, nothing is for free, including access to human furniture. They get 'their spot' in the house (in their pen where they can still see the others) where they can feel safe and not worry about others bothering them. I build up communication and confidence by starting (or going back to) basic commands with every meal (with food sitting at their feet): sit, stay, leave it (a VERY important command), look at me, eat. When possible, we go for walks together, one human holding the newbie, keeping him or her separate, yet close enough to be aware of the others. The outside world can be a good distraction to keep them from focusing on hating the other. When I introduce (or reintroduce) them to other dogs in a securely fenced yard, I start at times that aren't so exciting - quiet times, nap times, etc., not around feeding time or when we have visitors. It is important to not make a big deal about anything. Have the right thought in your head, but be prepared for a less than perfect response. Expect small improvements each time, but make sure she is set up to improve. Go very slow. Don't rush it. That is when failures happen.
I hope you get her under control quickly.

I'm sorry he has to deal with this.

Has your vet checked his ionized calcium level and PTH level? Has kidney function been checked? Depending on the parathyroid function and the cause, there are a couple of options to manage his condition. How well that can be done depends on how early it was caught.

I have seen parathyroid function affected by other health conditions including chronic kidney disease or Fanconi Syndrome and nutritional (calcium) deficits, particularly when young.

What did they do for her when she got into the tomato fertilizer? Has she had recent bloodwork? How complete was the bloodwork? Did it include phosphorus, IONIZED calcium, Vit D and/or PTH? The fertilizer may have thrown parathyroid regulation out of whack. An elevated phosphorus level will squash an appetite.

It might be time to consult with a vet specialist. I hope you find a solution quickly!

Lots of good input already. My thoughts are that he needs confidence building. Very short (5 minute) training sessions to work on commands like sit, stay, leave it, look at me, come, touch, etc., a few times a day, definitely with every meal, should help. Once he does a command reliably with you, have the kids work on the same command. This gives everyone a clear reason to praise him while building communication. Children can bring a lot of energy into a home which can be pretty intimidating when one is a little guy recently separated from mom and littermates. Having them participate in his training in a supervised manner will help them to appear less scary.
I prefer to leave soft rock or sitcom TV on almost all of the time. It helps to avoid a lonely silence while taking the attention away from complete focus on whatever I'm doing. It also masks the sounds of school kids walking by, squirrels running through the trees, other dogs, etc. If I don't leave on something to break the silence, my guys hang around on edge, waiting for the slightest noise to spur them all to jump up and run to the window with hackles raised.

Like others, I am not a huge fan of dog parks and have had an unpleasant experience with an unpleasant GSD owner who felt he should kick all three of my dogs who were trying to get his on-leash GSD to play in the off-leash park. I still value dog socialization, though. There are other ways. You might visit a local dog club to meet owners who might agree to go on walks with you and your b in neutral territory. You can walk in the area of a dog park and again, perhaps meet an owner who is willing to go for walks. After making friends with a buddy outside of the park, meeting at the park when there are few other dogs around for off leash play may make the park intro pretty simple and less intimidating.