GFS is a very clumsy way to organize backup, and only required if you have incremental/full backups.
Proxmox Backup Server give you automatic, fully duplicated backups, so you do not need GFS at all
(This is a great feature, not a deal breaker).
Why is "/var/tmp" a CIFS mount?
> INFO: filesystem type on dumpdir is 'cifs' -using /var/tmp/vzdumptmp281683_122 for temporary files
I would try to use a unix filesystem there (xfs, ext4, zfxs, nfs).
I think you may have confused hostname syntax with email address syntax. So the correct hostname (FQDN) would be "HYPERVISOR-03.oakhills" instead of "HYPERVISOR-03@oakhills". Please try to reinstall.
AFAIK Dell S300 Raid are fake-raid controllers, i.e. there is no hardware-raid support. Instead, they just store the configuration for some software RAID implementation (completely useless).
So you can simply use ZFS RAID instead.