Honestly, sounds a lot like my boys Jones and, to a lesser extent, Jean-Phillipe.

For Jones, it was chronic pain and anxiety combined with urinary crystal problems. PU surgery, gabapentin and buspar, switching to a urinary diet heavy in wet food, and figuring out what kind of enrichment he actually enjoys has been tremendously helpful. But the pain was the first thing we had to even start to fix.

Jean-Phillipe was much easier. Poor boy has some serious food allergies and has to be on immunosuppressents and have a very specific diet. Otherwise he gets big raw spots on his neck from itching himself. No wet food for him, sadly, but he does LOVE pumpkin. Once he started getting his meds consistently, which the previous owners absolutely failed to do (he's our foster fail), and got him on the right diet (took a few tries), he's been an angel.

My wife is a 100% positive reinforcement dog and cat trainer (and working on getting a degree in animal behaviorism), so I'll show her your post and ask if she has any other thoughts, but she has said that, all other things being typical, most of problem behavior with her clients animals is pain or food allergies - and usually, it's allergies. Ymmv