One of the problems is that GitHub's UI and workflow isn't very good for this in various ways (can't review commits, can't really diff with previous after amend

One of the problems is that GitHub's UI and workflow isn't very good for this in various ways (can't review commits, can't really diff with previous after amending commit). So as a rule, I tend to stick with "1 PR == 1 commit", except when there's a compelling reason not to.