Looking for best options to implement custom signup flow

We are woring on a migration project for a portfolio management company that has custom signup process (similar to ID proofing). App first collects the user details and unit holder number to validate against a registry system before letting user create an account.

We would like to move this signup process to Auth0 for a better attack protection by building a similar signup form that can validate the collected details against an API. Upon successful validation navigate the user to set a username and password, verify mobile and email attributes.

After thorougly going through the documentation we understood that Auht0 can’t render Forms for pre-user-registration trigger. The approach we are thinking is,

  1. Customize the signup form by inserting custom fields
  2. Create an action to call downstream API

What we really don’t want herte is collecting the username/email address attributes in the same form until the backend validation is successful. Also we are currently doing a lazy migration which requires dedup checks against another backend API.

Any advise on how these type of usecases can be handled with Auth0?