But I've also always struggled with clear-cut categories like that, feeling like a completely different person at different times of my life, [...]

I hear that. I think a spectrum/scale is probably more helpful than a binary A | B divide, and yeah also heavily context dependent. Like even really severe introverts usually have some close "inner circle" people who they don't find draining.

I also relate to feeling alone in a crowd, where you can sort of spectate the socializing without actually being the focus of it. Or just being in an anonymous crowd so large that you feel alone (the crush of humanity--the thing I miss the most about living in s big city).

Anyway, not that I'm looking for universal metrics, but just to say that most introverts probably aren't 100% that way -- they might just need every other weekend to themselves or something.