From our call on 4/29/2020, this diagram shows that NIEM and FHIR already have or will have the same underlying data model format, RDF and Web Ontology Language (OWL).  This allows for quick mapping, structured federated queries, and self-defining data that can easily be validated over the web.  Convergence is already well underway.  The source file for the diagram shown during the callis hosted here.