It’s most useful for something like /home so you have a full backup of your home directories without having to worry if some app has dumped its settings on a folder you don’t have marked for backup. But also backing up / is useful because it gives you an easy recovery in the event of say disk failure. Just restore the entire system from a backup.

• What’s a good NAS OS to install?

TrueNAS Scale is the go to. Unraid is another popular option.

• Any fun things I can do besides plex transcoding with a 1080 GPU?

Local LLM. Look up Ollama.

• Would it make sense to run a Pixelfed/Mastodon server off this guy?

You could. That could potentially use a lot of space or be very annoying you having to manage and moderate the instances.

• Can I run a RAID on it without buying a separate HDD bay?

What do you mean? Are you talking about a hardware RAID card, or can you physically stuff more than one disk drive into the chassis? For the first, it’ll depend on whether it has any open PCI Express slots. For the second, what do you see when you open it up? Are there 3.5" or 5.25" bays open?

Other than a Plex port forward, I have zero experience putting services out on the public web (but would like to learn!).

Wanting to learn is an admirable goal. I’ve not done it myself, but the Linux Upskill Challenge might be a good place to start. Either that, or figure out something you might want to host yourself, then come back and ask for input when you run into trouble or have a question.

You must have had a real sweetheart deal on VMware then. Proxmox is cheaper than VMware even under the old pricing. You also don’t have to buy the “Standard” subscription. There are cheaper ones.

That’s right. So on the top backplane, you’ll connect the Oculink ports to the Oculink outfitted HBA. One port per drive.

For the bottom 8 drives, it looks like you’ll have one miniSAS HD connector per four drives, plus another for the rear bays. I initially thought they were plain SATA and would go to the motherboard. But it looks like you’ll need a third connector - so you’ll want a 16 port HBA (Supermicro AOC-S3216L-L16iT).

Reading through all the documentation I can, it looks like you’ll have the option to run all the bays as NVME or SAS disks. The controllers and layouts I’ve listed are for running four bays as NVME, and the other 10 as SAS.

If I understand the ports you have - Supermicro AOC-SLG3-4E4T for the U.2 bays and a Supermicro AOC-S3008L-L8e for the SAS bays. You could replace the SAS card with a Dell HBA 330 as well. The Dell PERC cards that support NVME storage don’t appear to have the Oculink ports your backplane has.