Icloud Uploading Stuck. Does anyone know why it actually does this?
First, have you tried the suggestions in this thread: https://discussions.apple.com/thread/251061918

I have tried all the things that Apple support pages suggest, such as force-quitting Finder, rebooting, rebooting into Safe Mode, all to no avail.

If those don't work, I suggest you contact Apple Support directly on this one. I am working with them on what I believe is an identical issue of iCloud files not syncing from my Mac (macOS Catalina) to iCloud since the last few days. 

Go to https://support.apple.com/icloud and scroll to the bottom and click on the "Get support" link and choose "Update, Backup, and Restore". 

I have spent perhaps 3 hours with 3 different support people as the issue gets escalated to the support engineers. 

I don't know about you, but some files do sync up and back just fine, like those associated in iCloud Drive with certain applications (eg, Drafts, MindNode). I can create a text file at the top level of iCloud Drive and it syncs. But not for folder that I create manually. 

I have gone through a macOS Catalina re-install (via Recovery Mode) and that did not solve my problem. I am now awaiting the support engineers to review diagnostics information from my computer. When I have a definitive answer, I will report back on what hopefully solves my issue. 

I had wanted to comment on your post, rather than answer it, but I don't have sufficient reputation points to do that. 

Cheers, David