Can't help you with that "more than 5 mixer files" thing, never had that. Maybe another user with the same device could help there. An uninstaller isn't needed, just open the modules tab in Magisk manager and tap the recycle bin behind the regarding module and reboot.
I only used AIMP for PC a long time ago so I can't help you there too.
I'm a big fan of Poweramp, it was the first app I purchased.