I haven't been able to get two-way. I don't have a reliable way to get information back into iTunes, although I do have some leads to investigate via
https://samsoft.org.uk/iTunes/scripts.asp#ExportImport
But for one-way, here's what I'm doing:
1. I set up Syncthing
https://syncthing.net/ with my iTunes media library as a SEND ONLY, and the directory on my Android as Receive Only. This moves my music over, and is relatively fast.
4. I've got a perl script I've written that can parse a saved iTunes xml and a saved stats file from GMMP, and produce a new stats.xml
5. I've got syncthing set up to sync bidirectionally the directories where GMMP stores its stats.
My process is:
1. Run syncthing to sync music
2. Once music is over, initiate a rescan in GMMP to add the new music and update things.
3. Back up the stats
4. Go to the music directory. Copy the stats.xml to my working directory. Export my itunes library to itunes.xml in that directory. Run my perl script to update the stats, and copy the new stats.xml back to the sync gmmp directory.
5. Restore the stats.xml
Depending on how long it takes syncthing to see things, this normally takes 10 minutes once syncthing has gotten to the point where it will sync (since I start it up each time, about 20 minutes). That total of 30 minutes is FAR FAR less than it took with iSyncr and Rocket Player. Note: I have just under 54,000 songs in my library.
Daniel