I think I’d take a pass on the Wellness Simple Lamb and Oatmeal. It still contains peas, tomato pomace, chicory root extract and millet. If you suspect potato or peas, I’d eliminate them from the next food you try first.
I’d take a look at a different limited ingredient brand. Not because I don’t like Wellness, I do very much, but you’ve tried it and continued to have problems so something isn’t working. My favorites are Nature’s Variety Instinct LID, Canine Caviar, Acana (not the Regionals or the new Singles), Back to Basics or The Honest Kitchen Zeal. I’m not typically a Merrick person, but I see they have some new limited ingredient formulas. I’m also not a California Natural person, but they have a very simple lamb and rice diet that might work. And, while I despise Diamond, Canidae Pure would be worth looking at, too. I almost hoped it wouldn’t, but I found a variety that my dog with food issues does great on. Not all of those brands I mentioned exclude your suspect ingredients, so be sure to look before you buy.
Like I said, I unearthed my dog’s food intolerances though trial and error. Keep notes on every single food you try, the ingredients and how each dog reacts.
