Support for OpenID
What is the problem you are facing?
User login (and creation) should be possible with OpenID Connect (OIDC). By having a user pool on an authorization server this solves the problem of letting every user register another account. It is then possible to login with existing account.
What are the possible drawbacks or issues with the requested changes?
It should be possible to disable local user registration to make the app only accessible for users of the setup providers.
OpenID connect is a standard for authentication and authorisation based on OAuth2. See https://openid.net/connect/ for additional information on the protocol.