Which OS should my Jellyfin NAS run?

[TheDreadPirate]

Really, anything Debian based. Either Debian itself, or Ubuntu which tends to be a bit more aggressive with updates than Debian. A lot of NAS OS'es are built on Debian. Like TrueNAS Scale and Open Media Vault. TrueNAS Scale and OMV don't do anything that Debian and Ubuntu can't also do with additional packages. They just pre-bundle a bunch of other packages.

Of the bunch, if you aren't comfortable with the command line I would choose TrueNAS Scale.

If Jellyfin will be the only thing this box runs, and you are willing to learn a bit, I would install Ubuntu. Ubuntu is popular and because of that there are a lot of users to ask for help. There's a lot of documentation for setting up Jellyfin in Ubuntu. There is an official script to install Jellyfin with a single command for Debian/Ubuntu (bare metal, no virtualization). You need to add your NAS network shares to Ubuntu. After you get everything running, you would only need to use the command line for the occasional Jellyfin update?