OpenMediaVault VS BorgBackup

You might want to consider the previously mentioned MergerFS and SnapRAID, or MDADM and LVM for your setup. OpenMediaVault [https://www.openmediavault.org/ is a solid choice in this regard. Additionally, if you're planning to run something like Proxmox, you could look into deploying Starwind CVM on top of it. Pretty much like in this guide: https://www.starwindsoftware.com/resource-library/starwind-virtual-san-vsan-configuration-guide-for-proxmox-vsan-deployed-as-a-controller-virtual-machine-cvm/ .

You would better to install OMV instead. It's a ready to use fileserver solution with web interface.

So I'm working on a similar setup with an ancient desktop my in-laws were getting rid of. I installed OpenMediaVault directly to the hard drive (it's so old that I assume it wouldn't work too well as a hypervisor), with a 4TB external HDD attached. OMV supports Docker by means of a plug-in and I'm running a Jellyfin container with no issues at all. I'm still manually downloading everything to the HDD but *arr containers are my next step, as well as setting up a gluetun container to route all that through a VPN.

That is possible (it's automated to a certain extent). It also depends on the NAS OS. Unraid, for example, can work with drives of different sizes while using the largest one for parity. TrueNAS uses ZFS so you won't be able to just add a drive to your RAIDZ and expand it. There's also openmediavault: https://www.openmediavault.org/ that uses Linux Software RAID and you can add a new drive there but you can't combine drives of different sizes. And there's also Starwinds NAS that supports both Linux Software RAID and ZFS: https://www.starwindsoftware.com/san-and-nas. Plus, I would avoid SMR drives in RAID: https://nascompares.com/answer/list-of-wd-cmr-and-smr-hard-drives-hdd/

I'd put it like that: if you have a spare PC, then it's definitely worth trying. I would also look into openmediavault (minimal RAM needs): https://www.openmediavault.org/ and Starwinds NAS (supports both ZFS and Linux software RAID): https://www.starwindsoftware.com/san-and-nas

i use - and recommend - "borgbackup": for example with the "vorta" graphical frontend

* https://www.borgbackup.org/

* https://vorta.borgbase.com/install/windows/

just my 0.02€

I used this many, many years ago but switched to Borg[0] about five years ago. Duplicity required full backups with incremental deltas, which meant my backups ended up using too much disk space. Borg lets you prune older backups at will, because of chunk tracking and deduplication there is no such thing as an incremental backup.

[0] https://www.borgbackup.org/

Currently, I'm auto-backing it up with borg (push mode) through wireguard tunnel to NAS behind ISP's CGNAT. The borg takes care of deduplication in SQL file, so incremental update (even in append-only mode) is very small for PostgreSQL dump.

Borg 2 has been in development for nearly a year and a half [1] and may probably be released early next year, i.e., early 2024 (just a guess, seeing that even RC1 is not yet released and seems to have a lot of work to be done).

Does anyone know how Borg 1.x and 2 would compare to Kopia?

[1]: https://github.com/borgbackup/borg/issues/6602

You could use borgbackup.