Can't run a SOGo Backup

Yes, that’s possible. With automysqlbackup to save your databases you may just use the Nethserver backup data function, which does schedule an incremental backup.

Another approach would be some kind of virtualization(proxmox, vmware) to run your Nethserver and just backup/snapshot the VM image.

That’s right but why setting it up again when having it all in one incremental backup.

EDIT:

Hotsync backup may also be a solution for you to reduce disaster recovery time to nearly zero, maybe next to data backup: