Dietary concerns for ill cat

Okay. So our 12+ yo (unsure of exact age) cat has been to the vets back and forth. Yesterday he finally had his blood taken for his suspected hyperthyroidism. (no results yet, a week wait) Unfortunately the vet said he's got a very bad heart and kidneys and not to expect much for living much longer. I'm worried it might be CRF but I can't assume anything until the results are back. We were feeding him plain chicken and fish because he was vomiting and had diarrhea but since he seemed better we gave him some of his usual last night. Just whiskas etc. This morning he threw up some bile again. He seems happy in himself but I'm at a loss what to try him on. Vet advised hills, James wellbeloved, royal canin etc but I don't know what would agree with him and I don't want to buy so much expensive food for him to turn it down or it to not agree with him and have the bad stools and vomiting again.

Someone else suggested giving him Royal Canin Sensitivity Control wet food for his bad kidney/heart condition. I see the appeal in trying this. I've also heard of Kattovit, which has similar special nutritinal aspects.

What would anyone be able to recommend me or advise me of anything suitable for him?

I have had 2 hyperthyroid cats.

Someone will come along and offer info on nutrition, just wanted to send . Been there done that.

Good luck to you and your kitty.

I am not sure if you have the same there, but most pet stores here have small trial size bags for a few dollars or free so you can try the more expensive foods. They are not always out but if you ask them they have them (I wanted to try one they didnt have a trial bag for and the woman who owned the petstore opened a bag and gave me a sandwich bag full to try)