Here's a flowchart on how the 25th Amendment works written by a professor of constitutional law:
https://twitter.com/ProfBrianKalt/st...35825680048129
So yes, two-thirds of the House and Senate have to agree with the VP and Cabinet that the President is unfit for office...but they have 21 days to decide this. So if a majority of cabinet secretaries invoke the 25th, Congress can just run out the clock with Acting President Pence until Biden is sworn in on January 20th.