One thing I know is that Biden seems to be going into this tailor-made for the 25th Amendment. That is, unless the Dems are only interested in having a figurehead as president. They would get that with an impaired Biden. They would also get that with a VP such as Stacy Abrams, or even Michelle Obama, succeeding him.
Under the 25th amendment the VP can become the “Acting Presidnet” if the president is temporarily unable to discharge the duties of their office. Acting President has no time limit. Biden would only need to resign after January 20th 2023, then 10 more years of presidency for the VP. Acting president has the same power as the president. Anytime in his first two years he could be judge to have a medical issue and the 25th invoked.