Storage Setup for iSCSI/NFS servers
I think it's too many layers of abstraction to be tenable.
ZFS is fine. You don't necessarily need all of the data in a single namespace, do you?
You can expand ZFS zpools, if done carefully, but you should plan on your desired storage need and growth.
High availability in ZFS is also possible. See: https://github.com/ewwhite/zfs-ha/wiki