PlexAmp has some great flexibility but lacks offline functionality and easy navigation. Absolutely horrible at finding specific things to play via Android Auto, and completely unusable outside of network coverage. MediaMonkey seems to be the exact opposite, offline functionality is 100% and has beautiful navigation in AndroidAuto but the sync function only works at home and cannot be used as an online player at all without extensive setup (constant home VPN connection).

It's so frustrating to have to use two apps, but it seems to be the only solution for having both access to the full library when you have a cell connection and having plenty of offline functionality when you don't. I wish one or both of these app creators could flush out their apps, it seems with a bit of TLC one of these would be "the" solution but it's been years and neither have budged.