zfs can sort of grow by adding vdevs, as you say, however it's pretty wasteful due to the new parity drives It was much more efficient to expand the mdraid raid 6 and expand the btrfs onto that. The other backup server does use zfs (albeit the user mode fuse version). I set up that system in a similar way putting zfs-fuse on mdraid 6.
A bigger downside of ZFS, IMO, is lack of defragmentation and similar larger scale pool management. If you ever push a ZFS pool close to its space limit, you can end up with fragmentation that never really goes away, even if you delete lots of files. The recommended solution is to recreate the pool and restore from backup, or create a new pool and stream a snapshot across with zfs send | zfs receive. Not terribly practical for most home users.