First of all, I think you need to change vets. I know that sounds like a real Pain In The Ass! BUT my black Lab (yes, a dog but keep reading!) had a "mysterious" digestive/poop problem and I tried all kinds of different foods to help - none worked (oatmeal, adding veggies, whatever!) UNTIL I took him to another vet - on a Sunday, Banfield (Petsmart) because he had a something inside one of his paw pads. This vet was amazing, so kind, even funny, and he really liked her. So I went ahead and told her about his digestive problems. He'd get me up in the middle of the night - and it was mostly because his belly hurt. She told me to come back in a couple days, she talked to her fellow vet there, and they came up with a diagnosis AND a treatment. YES, it was an IBD, so yes, he was in pain, and felt like he needed to poop - but usually didn't. He needed an antibiotic and the one she found was called Tylan - usually given to turkey and pigs! Not expensive, but I had to get it from a feed store! Powder, mixed it in with his food AND chicken soup that I started making for him (well, I could eat it too). So it was tasty and good for him. A little bit of kibbles. His turnaround was swift and amazing.

OKAY, so even HUMANS need antibiotics and steroids if they have an IBD. IBDs can be Crohn's, ulcerative colitis, along that line - in other words, inflammation of the intestines. Here's a link (because please don't take my word for it - it's your BELOVED kitty! Best of luck to you and Skye! https://vcahospitals.com/know-your-pet/inflammatory-bowel-disease-in-cats