syncthing is a very useful utility for this. just install it on 2 devices, add a folder, then the files will sync over the network whenever both devices are on.

if you have an old phone or laptop lying around gathering dust you can set up syncthing on that and have it act as the always-on server. something simple like a raspberry pi with an external drive would do either.

every syncthing folder has a .ignore file where you can add patterns to reduce the overall size of the folder, which can be useful if youre going to try and sync your music library to your phone. its very basic but it can be useful in some cases. like adding *.flac would ignore all .flac files and only sync .mp3 does. or maybe if you have a few artists with very large folders you could ignore them and sync the rest. i havnt found a good solution to that problem yet tbh