For music on iOS I can't recommend foobar2000 [1] enough - and I tested ALL of the alternatives. You can import ANY folder (probably what OP is looking for) or use your "iPod Library". [2]

Personally, I sync my music via Synctrain (a Syncthing client). [3]

  [1]: https://apps.apple.com/us/app/foobar2000/id1072807669  
  [2]: https://imgur.com/a/7GVxB2y  
  [3]: https://apps.apple.com/us/app/synctrain/id6553985316

I was unaware that synctrain existed! That solved my last issue I had with my iPhone. I have been using keepass on various platforms to sync my password list, and with keepassium + synctrain I have my iPhone covered. Thank you!

I was unaware foobar2000 existed on iOS too, but the Windows version is my favorite audio player on that platform too.

But foobar doesn't scrobble unlike other alternatives