I've never seen a precise definition of stimming. Nor have I seen anything that talks about precisely how characteristic various forms of stimming are for autis

I've never seen a precise definition of stimming. Nor have I seen anything that talks about precisely how characteristic various forms of stimming are for autistic, otherwise neurodiverse folks, and neurotypical folks. I think the social model of disability is really helpful here. Regardless of what neurological commonalities there are or aren't, we have a set of social classifications of which potential stimming behaviors are normal or deviant. Tapping your leg is normal (sometimes annoying to people nearby, but normal), flapping is clearly deviant, and lots of other behaviors are somewhere in the middle. This is socially significant, regardless of whether it maps cleanly to some non-social difference.