I agree with the guy that said to put a separate sub panel up there.
While the cost of the heavier wire and the additional box is an extra
expense, it might be the same as running all that 12/2, considering
the price of wire these days.

This is the way I'm leaning now. Run 6/3 from a 60A 240v breaker in
the main panel to a sub main lugs panel on the 2nd floor, and use 12/2
to run 6-7 20A 120v circuits from that subpanel. I expect my max
simultaneous load to be around 70A at 120v.