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!