Thanks for the Swinsian suggestion, I've been looking for an alternative to iTunes for many of the same reasons... large masterfully curated library, I still buy my music and don't trust Apple not to screw it up or litter it with cloud crap. How do you handle syncing to iOS devices (where I've already switched to Cesium as my music player) with this setup?
I've been using Cesium too and it's very nice. I don't yet have an answer to your question. I'm experimenting with breaking free of the Apple "ecosystem" and figured music was the best place to start. I've been using Apple products for 34 years, but I'm beginning to wonder how much longer their products and services will truly appeal to me. I wish Apple would focus more on hardware, the OS, the UI, etc., and less on milking customers through half-baked "services". I think this is a telling sign of a growing weakness within the company, perhaps the beginning of the rot. They were always a hardware company, yet today they let the Mac stagnate while Microsoft of all companies bests them in desktop hardware innovation. Apple Music is pre-installed on hundreds of millions of devices, yet it can't get meaningful traction. I think Steve would puke if he knew about Apple Music. It's the return of DRM.
Anyway...
So I'm trying to use third party services as I'm no longer certain that my next phone will be an iPhone, or even that my next computer will be a Mac. Therefore I'm avoiding iTunes for the purposes of this experiment. I'm using Swinsian on the desktop or organize and manage my library. I have Google Music set to upload my entire library to Google. So when I'm on an iOS device, I'm now using the Google Music app for playback and not syncing my phone via iTunes. I can add albums to my Google Music library and see them across my devices, just not on my computer (unless I'm logged in to the Google Music web app, which is quite nice).
I view services like Apple Music, Spotify, Google, etc. as a great way to try music, but if I really dig the album, I buy it. I want the lossless audio files, plus streaming services completely screw artists. If you really care about music and the people who make it, you should buy the album. And this goes double if the artist is independent. More and more indie labels are selling lossless audio files on their websites and there are great sites like Bleep.com that have a huge selection of lossless audio from many artists and labels. Instead of trying to nickel and dime me with Apple Music, Apple would make A LOT more money if they just sold Apple Lossless files. Oh well.
Anyway, I don't want my computer library messed with in any way. I've had numerous minor problems with iCloud Music Library. Being a "collector", that isn't acceptable to me. While I don't believe iCloud Music Library has deleted anything from my library, I can't truly be certain as I have a huge number of files and I can't check all of them. But I have discovered duplicates and albums that iCloud Music Library wrongly reports as uploaded that I cannot access on my devices. In short, I don't trust the integrity of the service. So I'd rather keep my "collector" library in a one-way sync relationship with any cloud service.
So far I'm pleased with Swinsian's speed and responsiveness. Blows iTunes away. Time will tell if it's reliable and stable, but so far so good. And I'm surprisingly pleased with Google Music's interface, the responsiveness of the app, and sound quality, which I find better than Apple's Music app.