I won't comment on the merits of this project, but I put my entire media collection, including all audio, in Jellyfin, and I find Finamp (available for Android and iPhone) to be convenient player, especially that it has easy download of entire albums, artists, etc. and "offline mode" you can toggle on so you can random-play whatever you've downloaded, even in places with no phone reception.

I had a few bugs with Finamp, now I switched to the paid https://symfonium.app/ app on android, still with jellyfin backend. Which works great for me.

Thanks! Just tried Finamp and it works great!