Okay, this is exactly what I went through myself. Again, this is all from memory and was something I did four years ago, so I’m not sure what (if anything) changed with Plex. I wrote about it here:
https://forums.plex.tv/t/itunes-import-tips/327481
Long story short, the Unix-based versions of Plex would not properly import iTunes information because they didn’t support the a feature required to do this, but there was no good documentation about this (classic Plex). So what I wound up doing was creating a Linux VM with Plex, importing iTunes metadata there, and then copied that entire Plex library/database from the Linux VM to the instance of Plex TrueNAS jail.
I was just starting with Plex at the time, so I didn’t have an existing Plex library. If you do, you could potentially create a separate Linux VM, copy your existing Plex library to that VM, do the import, and then replace your original, TrueNAS library with the one from the VM that you imported everything into.
It’s a PITA process to do, but it worked for me. Or you could just continue running Plex in a VM. Plex’s Unix/BSD ports don’t get the same love that the Linux/Windows versions do. I wound up using TrueNAS exclusively to host files and then using another machine for my virtualization, which is where I had Plex running. If you have the equipment to do something like this, that’s the route I’d go.
