@4q2 Hey OP!
I've had the same issue when I rescued my 3 month old Rottie. He was on Nutriscience, we put him on so many foods and only found something that worked at about 8 months of age.
We tried blue Buffalo, blue buffalo wilderness, Purina pro plan, Fromm and Acana. The food I stand by is Bullymax. I live in Canada, so it got pretty expensive to ship, but if you live in the states, the food is affordable and has so many nutrients, with dry and raw food options. Don't be fooled by the name, it's not just for bullies (Even though my other dog, a pittie, adores that food.)
I've found that Open Farms to be quite comparable, with the same affordability, with allergen-friendly options, including dry and raw food options.
With my Rottie, I went with every sensitive and digestive track friendly option I could, so I included the Purina pro plan sensitive stomach wet food mixed with his food.
Thankfully, he's now 3 years old and thriving. The problems have since then gone away. But I feel your stress OP, I remember the countless vet visits, the late nights going to the bathroom with him, and the messes around the house because of how sick he was. Just hang tight!
Let me know what works for you!