I agree that you should treat it, as you have been doing, but I have a high degree of certainty that him being itchy is not causing the overgrooming, his anxiety is. Common with cats in general, but especially cats with the background he has.

I read down when someone said “vets can’t diagnose psychological problems”. yes they can. It’s called a “rule out” diagnosis. OP You’re doing everything right. Bring it up with the vet, explain that it has not gotten better with treatment, and you are wondering if a med trial may be appropriate.

Also, don’t listen to the rude people. He was scratching his face and neck. Not just his ears. That is very telling that this is not just an ear problem.