That's something I might do, but for music on the go, running a server somewhere is a huge amount of extra setup. It's also something i need to connect to. The battery saving from not running wifi/4g is huge. It's also one more thing that can break

Airsonic lets you precache albums with ease. It's really more of a wireless music management software which you only need to run when you want. Music is cached in a readable folder which any music app can pick up.

The version of DSub on F-Droid is unfortunately years out of date since the dev had to pull in some Google library for Chromecast support.

There's Ultrasonic (Subsonic client), MALP (MPD client) and Squeezer (LMS client) on Fdroid that are still active. Also Dsub might still work.

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.

https://en.wikipedia.org/wiki/Music_Player_Daemon

https://en.wikipedia.org/wiki/Logitech_Media_Server

That last fully FOSS DSub build does still work with my Airsonic server, yeah. Sorry my wording implied otherwise. It’s just out of date compared to the version you’d find on Google Play is all, but my phone is old too so I don’t mind :)