Create a private repository in Github with a fixed name and access it with Auth0. Upload the password database to the repository.
I have also recently been thinking about creating an open source password tool with Flutter. Found that there is already an AuthPass. So I gave up.
My idea was to store passwords encrypted in clear text on Github or GoogeDrive or OneDrive. Save the user’s Settings as well, Log in using one account from multiple platforms Automatically synchronize user Settings and passwords.