Right. . . plex is not supported on that device. . . . .and I think even if you got it to run. . .you would find a WORLD of pain due to the performance of the unit.
So music organization is a thing. I have broad subfolders for Genre; with subdirectories for Artists, with album directories underneath. Maintaining that structure. . .any number of programs will work.
Honestly. . . .day-to-day. . . I use an iPod from 2009. All the music is loaded, and my home receivers support AirPlay directly. When I use music at my PC (rare); I tend to use either Windows Media Player, or Itunes. . as the mood strikes. I have a HD with the music files generally at hand; if not those apps will read music directly off the NAS when running the PC.
Of course. . .movies can’t be stored on an iPod (or any tablet for that matter). If I am at home. . . I like to say I use Plex from the NAS. . . . streamed to a Roku stick. . . … .I have it all setup.
But honestly, I generally watch from a PC. . .which reads the files directly off the NAS. . . . OR if I am travelling (usually the case); I have a 4TB portable drive filled with movies. . . and I run a HDMI cable from the receiver/TV to a PC. Works dandy. (I gave up trying to stream Plex across the internet for a few reasons. . .it does work. . .but not my cup of tea)