Another +1 for vlc. We install most apps via the f-droid repository, partly because they're all free and partly because they tend to be the full versions of the apps. Works well on our cheapie (almost) no-name phones, and works especially well with voice commands. We have about 30GB of music in opus format, so quite a lot of music/files.
The one bit of additional work I did do was ensure each folder (album) had the correct album-art, so the car displays the familiar cover of the album. The ability to control music via voice commands is really an excellent safety feature.