icloud drive sync stuck

Go to the Terminal and type:

killall bird

Check if this fixes your issue. If not, go back to Terminal and type:

cd ~/Library/Application\ Support
rm -rf CloudDocs

Then immediately restart.

For 300gb of files, I had to wait an hour for anything to show up in the iCloud folder, and 4 hours for the sync to finish.

Source for this answer: here.

I found that use of a VPN can both cause and cure the problem. Certainly turning on a VPN midway through an upload has stopped mine, but I also found that turning it on can cause a stalled upload to restart. Of course, it might just be my internet service provider messing with me.

Under ~/Library/Application Support/, there's a folder called CloudDocs. Chances are this is the culprit. Delete it (or move it elsewhere) and iCloud will reset itself and stop looping and re-caching the same files over and over again.

1. Manually go through all the iCloud Drive files via Finder and look for all files with a 'cloud' (not-yet-synced) icon and drag or copy those files to a location outside iCloud Drive. (Perhaps someone knows of an automated way to do step (1).) (Simply dragging the files out and back in might also solve the issue; didn't think to try that, so if anyone can test that … ;)
2. In System Preferences:iCloud:options, uncheck the app(s) that had the 'cloud' files; alternatively uncheck iCloud Drive (in which case, iCloud Drive will reinitialize with all apps checked).
3. Then check the option:app(s) (or iCloud Drive) to restart it.
4. Move the files from (1) above back to their proper locations in iCloud Drive.