Is this legit?

Hey everyone.
So for a little background I have a 4F tabby cat. She is extremely food motivated for anything and a healthy 14lbs. (All in her tummy flap) a few months ago she started having pretty bad diarrhea including bright red blood (jelly like) mixed in. I took her to the vet and did blood work and a fecal sample. Both came back completely normal vet was confused and thought maybe she was just straining to poop and gave me probiotics for her. I thought maybe a sensitive stomach or food sensitivity so I started trial and elimination basically.
I found that Stella and chewys and nulo wet food were the culprits. Now she’s been strictly on orijen wet food and the Costco pate.
Today she again had a little diarrhea and blood. Maybe week since the last one maybe 1-2 months? I’m wondering are those prescriptions diets legit? For example the hills or blue buffalo? Money is not a factor in the food I’m willing to pay the expense of it but before I do is it worth it? I just want to help my baby.
Also I have continued buying and going the probiotics daily since this has occurred.
Thanks in advance!