BTRFS or ZFS sounds just fine to me, you could even create a small virtual disk for it on a host FS (less integrity, and one oight to snapshot the whole system

BTRFS or ZFS sounds just fine to me, you could even create a small virtual disk for it on a host FS (less integrity, and one oight to snapshot the whole system anyways, but hypothetically you could do this just for atomic snapshots as long the rest of the system is cattle and those snapshots are properly backed up / replicated)