Honestly self hosted Nextcloud, or WebDAV in general, is the best solution to sync Joplin. You fully control your data and you don’t have all the restrictions of OneDrive, Dropbox and so on.
For example, Dropbox may block a file if it triggers their copyright violation filter ; it seems that both OneDrive and Dropbox throttle connections, etc. To be fair the usage we make of their service is not in their interest, especially with E2EE, since they can’t do anything with that data.
Nextcloud on the other hand is as fast as your server allows, and has no restrictions.