I'm on the last level, and I've read that once I've completed that I'll be able to ascend and redo the old levels with full AP. I've skipped a lot of levels getting here; should I complete those before I move on or is it better to ascend right away, use up all the skip levels I've saved, and go from there?

You have to complete all levels (not just the last one) to ascend.

That makes that simple, then! Thank you.