kind of side note to this. supposing for the sake of argument that the best bet is harris. an obvious question that will be raised: if joe can’t finish the campaign then why is he still president?
The main reason I don’t think it will happen is precisely that Biden may step aside from the campaign but he absolutely will not step down from the White House.
You say that you are confident you can still serve as a good president but it's clear that a lot of his supporters have doubts about the next four years. So for the good of the country you're stepping aside to focus on the more important job of serving a President now.
It's not that hard.