If delaying the "write once" aspect of a volume (or tree) is OK, several of the above options offer read-only snapshots. Btrfs ro snapshots aren't writable even

If delaying the "write once" aspect of a volume (or tree) is OK, several of the above options offer read-only snapshots. Btrfs ro snapshots aren't writable even by root. They can be deleted or a rw snapshot can be made from them.