>> the federal government and most states enacted statutes in the 1920s and 1930s authorizing their courts to issue declaratory judgments <<
I scanned the Wikipedia article you cited, and all I could find was that Federal courts are authorized by statute to issue declaratory judgments only in patent and other intellectual property cases. So unless I missed some detail that you’ve found, there would seem to be no chance of their issuing a declaratory judgment about Cruz’s citizenship.
Federal Rules of Civil Procedure