Ubuntu Server 20.04 seems to have the same issue you warned about for 18.04. I tried ‘apt-get update && apt-get full-upgrade’, but that messed up my system royally! I tried to fiddle with it for over an hour before throwing my hands into the air and reinstalling from scratch.
It is technically in a Ubuntu Server 20.04 lxc container running under Proxmox, but it’s nearly identical to the real 20.04.
The issue was regarding a conflict between the installed mariadb and the mysql which was to be installed.