My kitten did not do well on Wellness cat food, what other good brand should I try?

I adopted my kitten, E, from a shelter when she was 3 months old and the first thing I fed her was dry Iams. After more research about what my cat should be eating (lots and lots of research) I gradually began feeding her a half wet/half dry diet that consisted of just Wellness product. Well, her tummy did not agree with this even after waiting it out for over a month. I decided to cut out the Wellness dry food and I kept her on just Wellness wet and her tummy was very appreciative. (Her stool was extremely loose/watery, smelly, and frequent prior to the switch.) Feeding her just wet food is just too expensive as she's a kitten (and they eat so much) after trying to do that for two weeks she was losing energy and the vet confirmed that she was underweight. So I put her on half dry/half wet feeding her Aveeno dry and the no grain wet. Wait a few months, the issue is not as severe but still there. She's now 8 months old and for the past 2 weeks she's been eating part dry/part wet Royal Canin Kitten food. Her stomach issues are so so much better but someday I'd like to feed E a food that was grain free food since cats are obligate carnivores. I just want to do what's best for her health, but I've hit a roadblock! Has anyone else had cats with any sort of similar stomach troubles? Or should I just wait until she's hit her 1 yo mark and feed her just wet? (I'd wait until she's older so that she doesn't require so much wet food.)