This is what worked for me (Big Sur, iCloud upload from 2013 MacBook Pro to 2013 iMac STUCK). I, first, did one last backup of my iMac data on Time Machine to an external hard drive. I signed out of iCloud from the iMac. That erased my desktop folders on the iMac but the documents were still present in the iMac's document's folder. Next I signed back into the iCloud on the iMac. The desktop folders reappeared and, in Finder, those folders showed the download icon next to each subfolder and file within the subfolders. I clicked on the icon of each desktop file (within the subfolders, about 500) which brings up a small window showing the progress downloading each file. I clicked only 1-2 files at a time (PDFs and audio files) to follow the download progress. This may sound tedious but the maneuver worked so far (performed 1 week ago) and, since, I have confirmed that newly-made files are uploading and downloading normally.