I also have an NW-ZX300 and a Mac. I have a 256GB SD card in the SD slot, and I keep my full Apple music library synchronised with my ZX300 using Bombich CCC (Carbon-copy Cloner). CCC detects the presence of the ZX300 as soon as it's connected to the Mac by USB cable. This ensures that the songs on the ZX300 are identically named to those on the Mac. Obviously without CCC you can just drag and drop files from the Mac to the ZX300 using Finder.
I export the playlist from Music as a .m3u file, and then edit the file paths, removing everything up to Artist (because the playlist is going to be saved in my top-level "MUSIC" folder on the ZX300. I save the file with a prefix "!pl " to keep my playlists at the top of the folder. I then add the modified playlist file to the music library folder on my Mac so that it will synchronise playlists to the ZX300, but again Finder will do the same thing manually. As already mentioned, the Music files and the playlists need to be either both on the SD card or both in built-in memory.
For the ZX-300 device you can also use .m3U8 format playlists (not sure about other models).
I use the same process to transfer my playlists to my Synology NAS (Synology told me it wasn't possible but for anyone reading this who's interested, they're wrong). For Synology drives, the differences to the process above are a) it only works with .m3U format, not .m3u8, and b) you need a text editor (I use BBEdit) that will allow the .m3u file to be saved with Unix line feeds - iTunes/Music saves playlists with legacy Mac line feeds, which a lot of devices won't support. But files saved using the Synology process will work fine on the ZX300.
Just remember that if you add or remove a song on the Mac playlist (or edit its properties) you will need to export and process the revised playlist as above.
