Google Play Music, apart from being a good local music player it's sync feature seemed to have been much sought after. With the app's removal, the local playback is addressed by apps like Musicolet but there is a need gap[1] for syncing local library and addressing it would be a great way to monetize such music player apps or perhaps even build a new one to address it?
[1] https://needgap.com/problems/179-upload-local-music-and-sync... (Disclaimer: I built this problem validation platform).
https://f-droid.org/packages/org.moire.ultrasonic/
https://f-droid.org/packages/org.gateshipone.malp/
https://f-droid.org/packages/uk.org.ngo.squeezer/
Also checkout Ampache. They support Subsonic android app after enabling some setting. Ampache has been going strong since 2001.
https://en.wikipedia.org/wiki/Ampache
Also checkout MPD (Music Player Daemon) and Logitech Media Server (LMS). All are FLOSS.
The battery life on the phone is shit, but since I only needed it for music, I could do something about that. I rooted it, added some automation: automatically launch music player and scrobbler on startup, activate airplane mode except when plugged in, add sshd service so I can rsync files to it, and turn the device off if music issn't playing for three consecutive checks (each 15m apart)
The only thing I need was a decent music player. I tried Musicolet, but it didn't suit me. For a start, it just doesn't work well on a small screen. Almost every piece of text was truncated. I also found that it didn't pick up media changes a lot of the time (eg sync a new playlist). I ended up going with phonograph because it's perfect on a small screen, has good controls and is really simple.