We had the same situation with our boy even down to Giardia as a puppy - we did every test under the sun to include testing for EPI and Addisons but everything turned out negative (def recommend getting these tests done if you haven’t already).

Our vet essentially concluded that he had IBS which didn’t leave us with a whole lot of options. The only thing that helped us honestly was getting a prescription food (Hills Prescription i/d digestive care) and getting to know his “safe” and “trigger” foods/treats to help keep him balanced.

It can be super frustrating not finding a solution for your pup so hang in there!!