That’s actually the guide I used! The only difference is when I tried to connect to adb normally, it wouldn’t recognize the device, so I had to use adb over WiFi (Make sure your computer and ONN are on the same local network, then type “adb connect ONNIPADDRESS” (replacing ONNIPADDRESS with the ONN box IP address which you can find in Network Settings on the streaming box).
It’s 10Mb, but here’s Fossify’s Music Player: Fossify Music Player on FDroid