So: after nearly 24 hours of frustration, my findings:
RapidWeaver sandwich files (.RWSW ) stall the upload.
I copied mine to a USB drive, deleted them from the Mac, and synchronisation completed.
Then UnSynch iCloud Desktop and Documents. All files become unavailable on the Mac, but thankfully remained in iCloud.
Then COPY the contents of the virtual Desktop and virtual Documents in iCloud, and PASTE into real Desktop and real Documents.
NowI have real files here , and copies in iCloud.
To use iCloud as a (non automatic) backup, I need to COPY on my machine, and PASTE into iCloud.
(If I drag a file into iCloud, it is removed from local storage.)