Cat with gut issues

Looking for guidance on how to best cater to my cat’s dietary needs.

We adopted a rescue kitten and from day 1 he had gut issues. We got him a PCR that showed no issues (some FCov antibodies but clearly long gone). Blood was taken, urine taken, fecal float. Everything clear.

Vet tentatively suggested it may be a protein intolerance and after a dismal failure with one brand of hydrolysed protein diet (PP HA) we went to the extreme of (RC) Anallergenic prescription diet. He stabilised very quickly on that and gained weight and had some growth. Gut settled. Vet encouraged us to use it as an elimination diet but not long term. There’s a lot of hydrolysed options out there and we aren’t sure what the best place to start is.

We know of one novel protein he can have (kangaroo) but the only cat food using it as a single protein is quite hard to obtain. He definitely reacts badly to chicken and turkey. We need something that is more available and safe long term. Bonus points if we can get our other cats onto it as well.

Do we go for (H) Z/D, (RC) Hypoallergenic, or Sensitivity Control or something else? What’s got a good reputation for cats with severe gut bleeds and diarrhoea from protein reactions?

Also are there any information sources I can go to for educating myself on the differences between protein intolerance and ulcerative colitis and other potential causes of similar gut issues?

Thanks in advance for any info you can share. I’m looking to do the best I can for this little guy. He’s nearly 2years old and I want his life to be long and healthy.