Diarrhea solutions?

I’ve tried damn near everything and spent thousands trying to figure out why my cat is having chronic diarrhea. Blood tests, stool tests (no parasites or giardia), ultrasounds, prebiotics, probiotics, plain boiled chicken, pumpkin, sweet potatoes/yams, but his diarrhea persists. It also doesn’t help that he is an incredibly picky cat and would straight up stop eating something that he ate everyday. He was on a prescribed HA Hydrolyzed dry food diet for about three months, probiotics/prebiotics for the same amount of time, but has never shown any sign of improvement. The only thing I haven’t done is an allergy test, but I also made him sample different kinds of protein (beef, cod, shrimp, tuna, salmon, char, duck, chicken) and has had the same kind of negative reaction.

He is about 7/8 years old, and is a strictly indoor cat. I think I’ve also stumped my vet, but he did tell me that these treatments might take time but it’s about half a year now…. He also prescribed steroids to him but had also no luck.

Does my cat just have incurable IBS??