There's lots of FOSS music players, but none of them have a volume slider / preamp. The Android volume slider is always either too loud or quiet so I have to make fine adjustments using the preamp in JetAudio. If someone could add that to an existing music player that'd be cool.
And one that could handle a decent sized library (10s of thousands). One of the few places I'm happy to pay for it, Poweramp will let you buy a licence without google services. If you've already bought it and are on GrapheneOS you can enable play services for (3?) days and it stays good after you disable play services. I seldom shill, but here we are...
You can do this in VLC, though it's not very practical. VLC's equalizer has a preamp slider, it's just not great if you want to change it all the time.