Ok, downgrading to 5.21 and then installing zfs 2.1 via Debian 11 backports is the solution here… Some pointers below in case anyone else runs into this…
https://linuxcapable.com/how-to-install-backports-experimental-repository-on-debian/
sudo apt install "zfsutils-linux" -t bullseye-backports
I probably could have just installed the backported zfs 2.1 and carried on with version 6.1 of lxd, but at least everything is working now…
btw, who had the wonderful idea to stop supporting zfs 2.0 when moving to 5.21/6.1? Seems like an odd decision to me!