What is a better point in an academic career to move to the U.S.: as an assistant professor or after tenure?
As someone who is just now making such a transition, my sense that it's hard to make the transition in either direction at any stage of your career.

My sense is that as a less senior faculty member, you'll have an easier time getting things going, because the expectations on junior faculty are lower in the sense of ramping up the group in the first few years. But whether or not you should wait is really a function of what you're looking for in your career.