Yes, you can backup a whole encrypted file /home/username.home but it would be slower.
I think you can not create Btrfs subvolume inside your home directory as the systemd-homed file.
I would not recommend use systemd-homed in Btrfs filesystem when you want to create multiple btrfs snapshots of the systemd-homed file, because each snapshot has a systemd-homed file 40 GB, then 20 snapshots would total 800 GB (20x40GB) usage.
systemd-home is a remote home-directory technology used for “roaming” between machines. (I think because i never used it)
Eg. they are not related to each other…
Backups can be done using many technologies, but the real important part is where they are kept.
The default Manjaro when using BTRFS is already using /home as a separate btrfs subvolume.
If you want real safe backup of your home directory you should place that backup on a different disk, no matter if it is a btrfs-sub-volume or not…
(Take not that i said “that backup”, and not just “your home directory”)
A user home directory is stored in /home/username.homedir and mounted to /home/username using bind mount on unlocking. When this method is used no encryption is provided. To use this mechanism provide --storage=directory or --storage=subvolume to homectl.