Is PHP 7.4 fully supported?
Not yet officially. If you want to track progress you can subscribe to the ticket at https://lab.civicrm.org/dev/core/issues/1496 and its subtickets.

If you find some issue feel free to report it in a ticket there.

The money_format issue might be something you could look at. If you're testing I'd look closely at how money amounts are formatted on the screen and in receipts - that kind of thing.