The only way to write something super sensitive like this imo is to have the back-end API that retrieves full credit report be essentially hard-wired to a user

The only way to write something super sensitive like this imo is to have the back-end API that retrieves full credit report be essentially hard-wired to a user access level of "FULLY_VALIDATED". That way no developer can come along five years later and accidentally grant the wrong access level or show the report to a not fully validated user. Put the security check on or as close to the thing being accessed as possible.