Hello,
I sync an entire folder ~/Nextcloud/Joplin and point Joplin to use it as the “filesystem backend”. When doing sync, Joplin dumps finished data there.
Right now I tried going for the WebDav backend to troubleshoot (i.e.: “does going pure webdav solve this?”) but webdav sync in Joplin is extremely slow right now, so I can’t test this scenario.