Some kind of back end storage area appliance to provide storage for VMs.
perhaps with choice of protocols, iSCSI, SAMBA, NFS, etc.
In the last few of months I been using the StarWind software, it works greet with VMs, and is perfect for your scenario.
Try first the free version, then if you founded useful, use the server version it’s even better.
www.starwindsoftware.com
Would it be possible to use the DroboElite on Proxmox VE?
I've tested with FreeNAS and OpenFiler and I must say OpenFiler is way better and faster! FWIW