If you want to run all of LXD to your second drive: boot into a rescue session, move /var/lib/lxd to /dev/sdb (or zfs partition), then create a symlink of /var/lib/lxd to your second drive.
Please don’t use symlinks if you want to do that. This is inherently unsafe and might mess with your setup. Use bind-mounts instead:
mount --bind /dev/sdb /var/lib/lxd