I bred a litter from my bitch who had the most amazing temperament.
i kept a female puppy from the litter and as soon as she got to be about 3 months old she became the most anxious and reactive dog. I was in shock as I’d bred her, she lived with 3 older dogs (all with fantastic temperaments). She lived in a loving, calm family home etc.
what I’m saying is that some dogs are genuinely just wired this way and it really isn’t the fault of the owner. It can be very difficult to get a handle on.
She was super anxious on walks, terrified of other dogs and would snap at them. Very fearful of new people coming into the home. But, thankfully she never showed any signs of aggression and I never thought she would attack anybody (she never has) she was just scared of everything!
anyway it took about 3 years but she did come out the other side eventually with a lot of training, accepting that she would never be able to be walked in public spaces where other dogs could run up to her etc. Very slow and calm introductions to people coming into the home and with time and lots of positive experiences her confidence grew and grew.
Now 10 years on, she still wouldn’t tolerate an unknown dog coming up to her (so I never put her in that situation). She is happy and accepting of anyone that we invite into our house. This is family, friends or complete strangers (workmen etc). Crucially she has always been fantastic with children and has always adored them for some reason so mercifully I never had to worry about that.
It’s a long and difficult road. Sometimes it works out and other times despite best efforts, it doesn’t. It sounds like you have tried so hard but to no avail. Personally I don’t think a dog should be drugged up to the eyeballs just to be able to exist but that’s just my opinion.
If you do decide to PTS it will be gut wrenching for you. But you cannot live another 10 years like this and it’s not fair on the dog either to live in this constant state of fear.
rehoming ‘might’ be a possibility but I would want to vet the people very carefully with full agreement that if it doesn’t work out the dog is to come back to you and then you can be the one to decide to PTS. Not sure if this is possible but it would be a deal breaker for me.