Gluten, dairy, and corn for me. Turned out to be MCAS. Supposedly more common than people would expect (and yes, it's an actual ICD-10 disorder), but a total bear to diagnose as it's a diagnosis of exclusion and that's very expensive and a lot of time.
I think a lot of people end up falling into the Candida/Naturopathic woo along the way due to the seemingly higher presence of provider empathy. Not that dysbiosis or supplements don't have a lot to do with helping some of the day to day, but it seems to be a downright strange obsession for some folks to the exclusion of actually treating the root of the symptoms (were it even possible, in some cases).
I think your smack dab on the sugar and refined carbs stuff too. That seems like it'd be good for everybody. I know I might be in a slim and tiny minority here, but I'm definitely in favor of a sugar tax on foods with added sugars/raw sugar/sugary substitutes that are sugar under a different name (fructose, dextrose, etc...) by itself...