I don't know what to do about my two cats

TL;DR: Both cats have had diarrhea ever since I adopted them, vet doesn't know what to do with them anymore. In the span of over a year I've tried about 40 different cat foods (and monoproteins), antibiotics, supplements, disinfecting the toilets every day, massages etc. and nothing works. Atp, I'm at a loss.

Hello, I'm feeling helpless with my situation regarding my two cats, so I hope I will reveive some help here.

As already mentioned, I have two cats (they are sisters, both about a year and 3 months old) which I adopted from an adoption center when they were very little. I was told that they have Covid and should be fine as time passes. Then, they started having diarrhea and they didn't gain weight for a long time. The diarrhea was so bad that they had to go poop immediately after eating. I was feeding them high quality cat food with good protein. I took them to the vet and they did an ultrasound on both of them (it showed that both had an inflamed gut). They also did a hemogram (blood work) on them to check if anything was weird but they were only deficient in vitamin b12 - I was prescribed a supplement and a veterinary diet for them, which both didn't do anything except make it worse because the cats hated the food. I have tried different monoproteins to feed them (turkey, rabbit, horse, beef, fish), none of which have made this better.

Over time the diarrhea got a little bit better (I don't know why, might just be due to a better immune system) but they still have diarrhea almost every time. They started gaining weight as they grew, but they are only about 6.2 lbs each. My vet suggested it might be an inflammatory disease in the large intestine (because my cats gained some weight). I also did a lot of research online and found out about IBD. But when I asked my vet about it, she said my cats were too young to try steroids or any medication that could help with IBD and that she doesn't think they have IBD (even though they show almost every symptom).

The vet has prescribed my cats multiple prescription diets over the past year (I've tried every different food for about 3-4 weeks to see if it might work), every single one has not made a great difference so far.

Right now, my cats have been prescribed yet another prescription diet (hill's food sensitivities original prescription diet adult). It's really expensive and to be honest, I can't really afford this in the long run, so I'm hoping there could be another solution to this.

Please help, I don't know what to do anymore... I don't know which information I should provide further, so please let me know if you need more information and I will try to provide it.