I use multiple media players, Play Music, Musicolet for local media, and a podcast player. I set it up so no matter which one I'm listening to, I can control it via shortcuts.
I then used a Webscreen bubble to control my media, so I can control it anywhere.
To get it to work, you will need to change all the media control actions to work with your own players (alas, each player uses a unique intent to play/pause/skip, so this step is necessary).
Control any music player with FAB 1.1
Floating button to control your current music player.