Build the snapshot with the below file

btrfs subvolume snapshot -r /path-to-sv/subvol /path-to-sv/subvol.ro

# send the subvolume to file, compress with parallel ZSTD & monitor progress
btrfs send /path-to-sv/subvol.snap.ro | \
    pv -c | pzstd -16 |  pv -c | \
    dd of=/path-to-external-backup/subvol.zstd.back

# delete read-only snapshot
btrfs subvolume delete /path-to-sv/subvol.ro

To restore subvolume from backup we run the process in reverse:

# read backup file and decompress the stream, redirect to temporary read-only snapshot
dd if=/path-to-external-backup/subvol.zstd.back | \
    pv -c | pzstd -d | pv -c | \
    btrfs receive /path-to-sv/


# make a RW subvolume 
btrfs subvolume snapshot subvol.ro subvol

# delete temporary snapshot
btrfs subvolume delete /path-to-sv/subvol.ro    

From here

https://superuser.com/questions/1396241/btrfs-imaging-a-volume-to-an-external-file

You might want to make this into a systemd timer to run at boot or before shutting down

https://linuxconfig.org/how-to-schedule-tasks-with-systemd-timers-in-linux