I have a Synology NAS and a few Sonos devices. Pulling these two nuggets of information together in case someone else needs to rework their setup. Previously I had all my music data on one machine, but since the NAS has plenty of space, it made sense to migrate the Apple Music library and then point Sonos in the right direction.
First you need to create a new library Apple Music
You can have more than one music library in Music. For example, you could have a library of holiday music that wouldn’t appear in Music the rest of the year. Or you could keep some of your music in a library on an external storage device.
Then these Sonos instructions illustrate how to get your old data over to the NAS library.
Choose in Apple Music under the File menu the ‘Import’ option. Select the folder where your music files are stored on your local drive. The Apple Music app will now copy all the files in the selected folder and subfolders to the NAS and update its music library. Bear in mind that all files will be copied to the NAS but not be moved, so the initial Apple Music library and local music folders will remain without change on your local drive.
This definitely can take a while. Unfortunately, Apple Music goes through a few different phases during import, only has minimal messages, and definitely no progress reporting. Oh well.