LG, there are some problems that can only be diagnosed through either a vet visit or dietary process-of-elimination. For instance, lab work by a vet can help determine if the pancreas is working properly; if it’s not, all kinds of problems can occur, no matter how high-quality the food or the ingredients used. (Same with a genetic inability to produce a particular enzyme.)
As far as food allergy or sensitivity goes, an attentive, well-trained vet can prove invaluable as one runs through the various allergen-eliminating diets needed to help pin down a particular food sensitivity. You may find yourself needing to prepare special food, made with a very restricted set of ingredients, by hand, to help nail this problem down. That wouldn’t necessarily be needed long-term, though, depending upon the sensitivity.
I’d suggest taking food labels and a stool sample from Link in to the vet, along with a packet of FortiFlora. Explain everything that you’ve done so far, what the results have been, and ask what route the vet would recommend to try to nail down the root cause of Link’s digestive issues.
Good luck!
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