You nail it as far as Harris stepping down goes. She has to be perceived to be doing it on her own, not being pushed (and not publicly expressing any lack of confidence in Biden), which means that job has to be there the second she leaves (which they can definitely arrange). Given those numbers, adding Whitmer (to nail the Michigan vote, and also polling best against Trump) would be the way to shore things up.

The real question, of course, would be how they avoid more of Biden being Biden.