EDIT: I am going to edit this to reflect that I HAVE NOT tried Joplin 3.5 which says: "
More reliable syncing and sharing
Syncing and sharing have been made more robust in everyday use. Joplin now handles repeated syncs more efficiently, avoids unnecessary data usage, and is better at detecting and syncing all changes, particularly when using WebDAV and S3 sync targets."
Until I have new data I will scratch this out:
In Joplin, I have never been able to successfully use an s3 instance with two or more joplin clients. It corrupts eventually. This is using a bucket for storage directly, not WebDAV.
From my research the best bet is self host, webdav, or some kind of file sync