The best way for Biden to quit would be pledge that he will resign the day after inauguration in favor of Harris. That way, the Biden/Harris ticket can remain intact for the balance of the election. Biden can let Harris run the convention and have the keynote speech. And select a surrogate Vice President whose name would be given to the Senate to appoint the day after Harris is sworn in as President. The beauty of this approach is that there would be no disruption of the current process of setting up the election and no convention fight.