I've been having issues with my S21 FE as well. I haven't been able to update at all wirelessly for the last year. I have to use Smart Switch, which is janky but works for me until security patches end at which time I'll be getting a new phone. Might be worth trying that (after a proper backup, of course).
Sounds like stacked updates. If your phone has missed updates/security patches, it will have to install every missed update in consecutive order until you are caught up.
If you can plug your phone into a PC running Samsung's Smart Switch software, you should be able to "jump" straight to the current version. Otherwise, you'll have to just install every update OTA until you're caught up.
