Upgrading Helios64 from Armbian Buster to Bullseye (see below) works as expected on my system.
However, I am using systemd-networkd and just a few services (nextcloud, netatalk, etc. and not ZFS)
EDIT: Upgrading Buster installations to Bullseye also works fine if you use network-manager, even if you have a bridge configured (using bridge-slave; binutils-bridge).
# cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 11 (bullseye)"