I can actually give you some good guidance on this. If you have a more holistic/crunchy pet store in your area, one of the best diets you can give them is Answers frozen raw, with answers raw goats milk, and their fermented fish stock. If the kidney issues are far along, a mixture of 50% goats milk to 50% raw (beef, preferably), and their daily recommended of the fermented fish stock. That’s literally the highest quality diet you can give them.
For a less high quality (and thus cheaper) option, I would still recommend replacing part of their protein intake with answers raw goats milk, and definitely the fish stock if you can swing it. Even in less therapeutic doses, those are two great sources of probiotics to aid in digestion as well as adding more liquid to their diet, which is job #1. Job #2, though it’s really job #1.5, is the quality of the proteins they’re eating.
Weruva steak frites is the best canned food for kidney issues, because while keeping moisture high and protein quality high, it also has low phosphorous. Which is job #3. If steak frites is still out of your budget, their chicken frickazee is the second best option.
These are all canned or raw foods. I guess job #0.5 is to cease any and all dry food. If your cat is on dry food, stop immediately. No cat should be eating kibble, but definitely not a cat with kidney issues. Dry food will actually contribute to kidney issues, if not just straight up cause them. As well as tooth decay, obesity, dehydration, diabetes, and a host of other issues like coat quality. Cats a desert animals by nature, so they’re used to getting their moisture from their food. They also eat until they get the nutrients they need, and kibble is so much filler that they will overeat, their dehydration will get worse, and their kidneys will suffer. Especially the lower quality foods like science diet and royal canin.
So no matter what, no kibble from here on out. Be wary of any of the restricted diet stuff like science diet’s because it’s not a complete meal, it’s for maintenance for a flare up in these issues. I mean, avoid science diet altogether (and royal canin. Just saw the subtext of the post). Dave’s makes a good replacement for this, and it’s cheaper because it’s not “prescription,” you can just get it on chewy (or better yet, your local independent pet food store).