- 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.
If you’ve got two 3.5" bays, you could do a RAID 1 (or a mirror in ZFS terms) with them both. This works very nicely with a small SSD for booting. My TrueNAS server has a 120 GB SSD in the M.2 slot that TrueNAS is installed on, then I have an array of spinning disks that forms the main storage array.
If you are planning any sort of play environment that you might want to keep (like a Pixelfed instance) I’d strongly recommend RAID just for availability in the event of a drive failure. But more than that, backups. They are of number one importance. Before you turn up anything of any importance, figure out a backup strategy.