, first, make sure everything is physically on your Mac
Nothing gets removed from your Mac unless you enable optimized storage separately. All of your data is always still on your Mac and it doesn’t even move from its storage location on the drive.
, turn iCloud Drive OFF entirely,
Absolutely no need for doing that. Waste of time. Does not affect a single thing.
As I noted above, every single bit is still written to your drive, exactly where it was before turning on desktop and documents. When you turn it off, every single bit is still in the same location on the drive.
All that happens is hard links to the files are created in the iCloud Drive folder in your library. The desktop and documents folders in your home are hidden by the Finder. All file references are still active in those hidden folders.
When you turn off desktop & documents in iCloud Drive, the hard links to the files that existed in your desktop and documents folder folders are removed leaving only the hard links in the mobile documents folder. When you then move the files from iCloud Drive back to your new, empty desktop and documents folders, hard links are once again created in those folders and removed from the mobile documents folder.
As they say in the movies, no bits were harmed during this process.
all of the wild accusations on losing data and everything else is just FUD.