One thing I somewhat miss from what you are explaining is all the aditional things that LXD gets you (snapshots using ZFS, image publishing/sharing, migrating containers between LXD hosts...)
But maybe some of those things are still doable (e.g. mounting a ZFS dataset as storage for /var/lib/machines/containerX)...
Haven't dealt with live migration, but mounting filesystems should be easy using systemd's unit dependencies.
Just drop a .mount file in /etc/systemd/system and set RequiredBy=systemd-nspawn@something.service and StopWhenUnneeded=true and the filesystem should be mounted before the machine starts and unmounted when the machine is shut down. See the manpages systemd.unit and systemd.mount for details.
One thing I somewhat miss from what you are explaining is all the aditional things that LXD gets you (snapshots using ZFS, image publishing/sharing, migrating containers between LXD hosts...)
But maybe some of those things are still doable (e.g. mounting a ZFS dataset as storage for /var/lib/machines/containerX)...
Thanks for your answer!