Agreed, but you would be surprised at what you can't find here unless it's a Toyota 🤣. took me a month to find a cv axle, and over a week to find 1 oxygen sensor. Gm is very scarce here I have only seen a handful in the 1.5 yrs I've lived here. It's a whole other world here. But I am going to see if I can track one down. I miss being able to go the parts store down the road and get what I need 😂😂Glad I can help. Unfortunately, knowledge alone won't fix a vehicle, it requires tools and parts, read $$$.
If I had to bet I would bet it was a malfunctioning/dirty EGR valve causing the negative trims. It is very easy to get to and remove. You can try removing it, cleaning it, and reinstalling it without a new gasket, but that might create more of a problem if after reinstalling it the gasket leaks. The EGR gasket is a very common inexpensive part which is interchangeable with many different GM vehicles. If you can acquire one it would be great.