Can't Substitute Previews for Originals in the Cloud

This is a last-ditch effort to fix my sync issue which two remote access sessions with Adobe customer service and advice from another forum has failed to resolve.

When I travel, I import my RAW files into LR Mobile on my iPad to do minor sorting, editing and sharing. When I get home, my original RAW files automatically download to LRC on my PC. This leaves the original files in the Cloud where they take are taking up 70% of my 1TB space.

What I want to do is free up space by removing the original raws from the Cloud and substituting previews which do not count against the storage space but keep them available in LR Mobile and Web. This should be a simple matter of unsyncing the files from the Cloud and then resyncing them. Unfortunately, whatever I do (or whatever Adobe customer service did), results in the resyncing photos getting stuck in the sync queue. The only way to stop this happening is to remove the photos from LRC and re-import them. Of course, in the process, all the edits are lost.

NB. There are no photos in the web Clean-Up, Deleted or Sync Issues folders on the Web. I have rebuilt the Sync Data. I am using W10. LRC and Mobile are latest versions.

The options I am yet to try are: i) saving the metadata for each file (thousands); or, ii) creating a new catalogue and importing the old one. But this will lose all the Publish Service data for SmugMug.

Grateful for any other suggestions to fix what should be a simple matter of LR ecosystem functionality.