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
replies(3):