I don't think you'd need a custom ROM. You could just factory reset it and only install apps that's you'd use for music. I can't remember if you can remove bloatware unrooted, but you could set up a clean home screen and ignore all the unnecessary apps.
Alternatively, I picked up an old FiiO X3 from 2016 for like $100. The battery is way better and the audio quality is fantastic. It's nice having a dedicated device that I can navigate in my pocket, and I dig the iPod Classic vibe.