I run a small android phone (with a 2.5 inch screen) as a music player.
While the phone itself is perfect - I've not really found a music player I like. Foobar2k's android port comes close - but it won't let me queue up music.
So my requirements are
Runs on android 6 (and frankly the phone is mostly offline so if its an 'older' less supported player with the rest of my requirements I'm good). There's very few options on the size range, and this device does what I need perfectly.
can handle a large music collection from an SD card - I've about 10k songs of various formats including Flac, MP3 and WMP
I need a searchable playlist of all tracks - I mostly play random music and sometimes decide I want a song. Foobar2000 does this ok
Need to be able to queue up tracks to play next and be able to do so with multiple tracks. The desktop foobar2k does this, but not the mobile one. I think the old google play music did too.
Needs to be usable on a smaller screen. I don't need perfect - but if its something with a busy busy UI, Its going to be a pain.