Not judging, just wondering- have you tried a behaviorist? Would that be an option financially? Obviously the medication isn't really doing what you need it to, but it seems like these are things a behaviorist could help with, especially since he is good about giving warning signs when he is upset or anxious.
I think it can really vary a lot. It would be worth asking your vet. I don't know a ton about behaviorists, but your vet should have some recommendations for people. I would also just call around and get some pricing and share what's going on. I do think it's worth a try, especially since you seem pretty unhappy about the idea of rehoming him.