Just to preface my comments I am a small animal vet… I have only turned away 1 behavioral euthanasia. I put down one of my absolute favorite patients after he bit his owner on the head\face (140# doberman). Yes the vet has to be able to go home and look themselves in the mirror BUT you seem to have taken some extreme measures to mitigate circumstances and I would have very little qualms in recommending euthanasia in this situation.
Behavioral meds will maybe help but honestly once that bond is broken there is little to fix it. You will always expect these reactions and never fully engaged or relaxed with him.
You say it’s different than them being in pain and putting them down but imagine living a life of constant anxiety and worry. He is living in a world he cannot grasp. In many of these dogs you are providing a way out of a life of misery and anxiety.
Hugs to you, this is an impossible situation to find yourself in.
If you truly want to exhaust all options please seek out a boarded veterinary behaviorist. https://www.dacvb.org/search/custom.asp?id=5985