Plan would be simple. Resign from the Presidency for health and personal reasons. This would immediately promote Harris to the Presidency, who could inherit the campaign cash and pledged delegates, and begin to sharply campaign on continuing the Biden legacy and attacking Trump with every skill at her disposal.
Wouldn’t be perfect, but it’d avoid a big, chaotic, inter-party fight with no prep time, and would preserve the sense of incumbency. Harris could even pick an actual progressive as her running mate, maybe a Katie Porter, and help shore up some support on that flank as she fights for the moderates.