It really has little to do with most legislation. Hell, he might not even whip his caucus on a lot of what Biden wants to do. Although he might want to filibuster on raising taxes.
What Mitch really cares about, what he views as his legacy, is his packing of the Federal Courts with right-wing zealots.
What he does not want to see happen is anything that might reverse that, which would probably require legislation expanding all of the number of Judicial slots — at the District, Court of Appeals, and Supreme Court levels.
Because he has himself nuked the last filibuster on appointments, that for SCOTUS.
As long as he thinks he can get away with it, he will absolutely block even letting the organizing of the Senate to proceed, calculating that most ordinary folks won’t understand his obstructionism, and the media will at a minimum find a way to “both sides” the issue. He is depending upon one or more of the 6 Dem Senators who have expressed opposition to getting rid of the filibuster holding fast.
Just my perspective.
Make of it what you will.