I would consider keeping/adding a member(hidden, non-voting) and keep it on ZFS as it is so versatile for ZFS snapshots.
XFS on top of LVM2 can give the a similar experience(in my opinion inferior) to ZFS. It would allow an immediate snapshot and allow you to mount the snapshot and copy it off.
Mongodump, even the manual will dissuade you from this method
mongodumpandmongorestoreare simple and efficient tools for backing up and restoring small MongoDB deployments, but are not ideal for capturing backups of larger systems.