I began using Joplin now too. Thanks for your posts above.
For both my Linux computer and my /e/OS phone, I use:
Synchronization: Nextcloud or WebDAV
Nextcloud-WebDAV-URL: https://murena.io/remote.php/dav/files/USERNAME@e.email/Joplin (Create this folder on murena.io beforehand!)
Nextcloud-username: USERNAME@e.email@murena.io
WebDAV-Password: The single use app password (since I use 2FA)
USERNAME
is your name before your @e.email/murena.io
address. Also shown above the single use app password field in the settings.
I hope this helps someone!