Originally Posted by RGW
The best food we can find based on our limited options is Purina One Beyond for dry food and Fancy Feast Classic for wet food.
Can I make a suggestion? Is there any possibility of ditching the dry food altogether? The Purina kibble still has ingredients that could potentially be problematic for a kitty with an overly reactive digestive system. There's soy, yeast, egg, and quite a few grains. If you could go 100% wet, and perhaps even a raw diet if that's at all an option, I think your cat might really benefit.
Hazelrunpack's suggestion of probiotics is excellent - look for something with a high number of colony forming units (CFUs), in the billions, and a wide variety of strains, preferably dairy-free; and also pick up some slippery elm bark powder. I find it works better than pumpkin for firming up loose stool. Just a pinch (1/4 tsp) twice a day mixed with wet food is usually all it takes.