No, it’s essential to the flavor.

drop back and punt with my old i5-4590 tower

This one. It’s old, but with RAM and a GPU you’ll be able to handle streaming/transcoding.

The problem with Dell/HP/Lenovo etc. PCs, especially smaller form factors, is that they’re non-standard, as you found. And not just the motherboard mounts, but also the power supply connectors, and often other stuff too. And any feature found on a typical board, but not used in that particular model, will be missing, like multiple SATA ports, again as you found.

You could probably get a newer desktop for not much money, too.

apt/dnf

They also need to run a VPN client.

Because you’re not putting bare jellyfin on the internet, right? You shouldn’t be doing that for most services in the first place, but doubly so for something that has a bunch of APIs that require no authentication: https://github.com/jellyfin/jellyfin/issues/5415

I didn’t know why anyone who has Plex would pay for a pass. The whole time I used it, I never felt any need for additional features.