First things first...sending tons of virtual hugs for you and Lenny. Now for the "fun" part. You can do this and it will be far less stressful coming from someone Lenny knows loves him.
If you don't have a grooming table, find a comfy spot for both of you. I put a cozy blanket over the guest bed and tied the leash to the bedpost. Use a muzzle. Follow vet instructions as reviewed above.
My boy "Rock'n Rowdy Rascal of Thor" went through this in 2016. He growled, but had never bitten anyone. The muzzle was a preventative measure. Once it went on, he relented and snuggled up to whoever was treating him. Rascal crossed the bridge in 2017. By the time we got the Cushing and cancer diagnosis he had deteriorated too far for treatment.
You are 2 years in and that lends hope. We are here anytime you need to a shoulder. Your Basenji family in this forum is a treasure of knowledge. They know more than many vets.
Prayers lifting...
Cynthia