Yes I’d say Dropbox is more reliable than OneDrive. Certainly Dropbox would be better than any free WebDAV providers, but if you self host or find a good WebDAV provider with servers based geographically close to your country, you would likely get faster sync with a WebDAV provider than Dropbox (but not as fast as Joplin Cloud / Server).
can i simply copy the folder to dropbox and try to sync
No, don’t do that. See How to fix synchronisation issues and start over
IMPORTANT: Make sure you have a working jex backup before you delete anything. To test the backup, create a new profile on the desktop app, import the jex and then make sure everything you want is there