Fair enough. I sort of fixated on your last post and forgot the point you made in your first post.
It is a while since I used a Nextcloud server for sync but an option is to try the WebDAV sync connector option rather than the Nextcloud one. I used to use that when I had a Nextcloud server as I found it more reliable. Of course this is unlikely to work if Tab Digital have done something that causes the OpenSSL mismatch between client and server. (Your other post).
The other option is, as you say, to try to use the Nextcloud client (assuming it does not also suffer the same error) to do the work instead. In that case you use file system sync to point Joplin to the folder being synced by the Nextcloud client.
Always backup first, just in case.