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.

For me what worked is going into my Utilities, opening the Activity Monitor. Then look for a process called "bird" and force quit. No files deleted or moved, it just restarted the upload process.

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.