So … after changing my revision history to 1 day and making sure that there was no revision anymore in the DB (as shown by Joplin’s synchronization status and highlighted in my previous comment), I’ve created a bash script to remove all the orphaned revision files.
This works for me because I’m running Ubuntu on my computer (i.e. I have bash), I sync Joplin to Nextcloud and Nextcloud is also synchronized locally on my computer. In other words everything’s performed locally on my computer. Once the cleanup’s done, it’s automatically synchronized back to Nextcloud (I assume this would work similarly for Dropbox for instance).
So far so good, Joplin sync folder now shows the expected number of files (38 in my case) and Joplin is synchronizing without error.
If you’d like I can share the script but it’s tailored for my needs and my configuration. Thus you’d be using it at your own risks and you’d better tweak it to your needs. Still it could help you start quickly if you have a similar issue.
Anyway @laurent there seem to be a bug with the revision cleaning in the sync directory (at least for me).