I’ve just posted Upgrading Sympl - Sympl Wiki which summarises the notes from a good few Sympl 10 to 11 (Debian Buster to Bullseye) upgrades and the relevant fixes needed after to bring it fully in-line.
It only covers Mythic Beasts servers at the moment (as I don’t have access to a huge range of hosting providers Debian images) but if you’re reasonably experienced you should be able to fill in the relevant blanks.