I have the same issue and did go ahead and try turning off Sync Library. Unchecked the box, hit Okay, closed the window. "Loading Cloud Music Library..." was still showing up (both bottom left and at the top of my screen), so I opened Settings again, and the box had re-checked itself! Any further ideas? I left my laptop on all night to see if it just needed more time to upload everything, but every song in my library already has the "The song, album, or playlist is downloaded to your computer" symbol next to it.
When I go to check Music on my phone (logged in under the same Apple ID on iOS, Sync Library turned on), some songs have downloaded, but a large percentage say "This item is not synced across your devices," and then urge me to go turn on Sync Library on my Mac.