Support PassLink for Setup?
Passwords has this PassLink feature which displays a QR code to be scanned by another device with a Passwords extension installed to set up the sync automatically (check the "Apps & Extensions" page under "Settings" of the Passwords web app.
I think this would be a very cool way of the initial setup to work also for this app.
The only description (but this is more a user how-to) I found on this feature is hidden here: https://github.com/marius-wieschollek/passwords-webextension/wiki/setup-firefox-desktop