I'd participate in that study if it existed

Same with a lot of other health problems. We've had so many people who have gotten it, many of whom probably didn't think much of it at the time or didn't even know they'd been infected. That's pretty likely to have population-level impacts.

We know it can affect cognition and increase risks for a variety of health concerns, but somehow the idea that those individually-recognized issues might start showing up in population-wide trends doesn't seem to cross people's minds. Instead we get stories wondering about why everyone's gotten angrier, or worse at driving, or why younger people are having heart attacks. Certainly those things could have entirely unrelated causes, but the idea that it's actually mass infection with COVID isn't even brought up and then rejected, it's simply ignored.