Plexamp Headless is almost certainly the best option, however it’s a Plex Pass only feature. It’ll run nicely on Raspberry Pi OS (or any other Debian-based distro as far as I know).
Just be aware that a lot of the best parts of Plex for music are enabled by Sonic Analysis (also a Plex Pass feature) which is unfortunately not able to run on ARM-based Plex Servers (including Raspberry Pi).
