Alright. I think I've finally figured out a quasi-convenient syncing solution that allows me to buy music on Google Play Music, sync it to my SD card and PC, and play it using whatever app I want on my phone and PC. Brace yourselves...
1) Move all music on PC to Google drive, and change iTunes library folder. Then...
2) Download any music purchased on Google Play Music to SD card using Music Mate. This is a search and select process. Once selected, the song/album is downloaded to the Music Mate folder on the SD card, and converted to something other apps can read (including cover art and meta data).
3) Move the music from the Music Mate SD folder to my Music folder using Folder Sync. I can do this manually if I want to immediately play the music or add to playlists with my music app. Alternatively, I can schedule it to happen automatically every hour (or longer).
4) Add new music to any relevant playlists in Rocket Music Player (my favourite)
5) Sync new songs & new/updated playlists to and from Google Drive. Again, I can do this manually, but I'd say I'll just schedule it to happen every hour. (From here, Spanning automatically backs them up, cloud-to-cloud.)
6) Google Drive syncs all files with my PC via the Google Drive desktop app. (I then back them up to my local USB drive and cloud backup using CrashPlan.)
7) I can then play my music on Groove or iTunes on my PC, and buy with iTunes or Google Play Music, and all purchases will be synced. I can also update my playlists in Groove and my changes will be synced, or I can import the playlists into iTunes and update them there, but that's a right pain in the arse. I think I'll only be using iTunes when I want to sync the old iPod Classic.
And to think I wouldn't need to do most of this if Google Play Music would simply treat music as it treats photos (and sync to Google Drive) and allow more than 1,000 tracks per playlist.