I can suggest a few popular Android Auto-compatible MP3 player apps that you can consider as alternatives to MediaMonkey. Keep in mind that the availability and performance of these apps may vary based on your device and Android version:
Poweramp: Poweramp is a highly customizable music player known for its powerful equalizer and extensive format support. It has an Android Auto interface for easy control while driving.
Pulsar Music Player: Pulsar is a lightweight and user-friendly music player that supports Android Auto. It offers features like gapless playback, Chromecast support, and a built-in tag editor.
Rocket Player: Rocket Player is a feature-rich music player with Android Auto support. It offers a clean interface, custom playlists, and options for managing and syncing your music library.
PlayerPro Music Player: PlayerPro is another well-regarded music player with Android Auto compatibility. It includes features like a 5-band equalizer, tag editor, and support for various audio formats.
BlackPlayer: BlackPlayer is a minimalist music player known for its clean design and customizable themes. It has an Android Auto interface for in-car use.
Shuttle Music Player: Shuttle is a simple yet functional music player with Android Auto integration. It supports gapless playback, lyrics display, and various themes.
Google Play Music: Google Play Music used to be a popular choice, but it has been replaced by YouTube Music. However, you might still find Google Play Music functional if you have an existing library.
YouTube Music: Google's official music streaming service, YouTube Music, also supports Android Auto. It allows you to listen to your own music library in addition to streaming songs.
Before making a switch, I recommend trying out a few of these apps to see which one suits your preferences and needs the best. Make sure to check their compatibility with your Android Auto setup and any specific features you require, such as library management or equalizer settings.