Is there a straighforward instruction how to make it work on Debian Bullseye? All instructions I find seem to use the git method, and also there are warnings to mix the methods.

Installing mailman3 with Debian methods was straightforward, and worked out of the box. Though it purged my mailman2 installation without warning, that was a hard blow. Had to reinstall all my lists in a night session. Also lost the old archives and can't get the new get running.