Thank you for clarification - for the Identifier first authentication there is indeed also a sign up button users can choose.
The Pre-User Registration Action - just to confirm - serves for the db and passwordless (not enterprise) connections.
Leaving the topic open so possibly the community can help.
What comes to my mind is to conditionally “force” users with the domain specific email to be redirected right to the Azure AD login page when clicking the login button in the SPA, but I believe the logic for this would require to be applied on the app side and leverage some localstrorage data of enduser or some additional step for the user before redirected to the relevant login page.