The general impression I got from my research is that some of these tests can be useful, but only when used in conjunction with other observations and case history. I do agree that some can be pretty scammy though - especially in their claims of the things they can fix. So like the parent poster, I'm really eager to see how these actual science-backed and FDA-approved solutions do for people too.
Also, stress and burnout are certainly a big factor in depression and fatigue, but these autoimmune problems are very real and can also have similar effects. I personally have a diagnosed autonomic nervous system disorder that apparently contributes to both the autoimmune and gut issues I deal with, as well as my severe fatigue and impaired mental state. I don't know that those areas - the gut, energy, and brain - are directly related on their own (the doctor I'm working with thinks they are), but I'm happy to try anything that might help any of them, and see what happens.