A health platform I helped build was open sourced[0] (the apps built on it are closed source and deployed in NHS trusts). Feel free to dig around for any inspiration :-)
The architecture was that that was a backend for apps, and the individual apps would host a stateless BFF service to translate the backend into what they needed, and then the web/mobile apps would talk to that. HAPI FHIR was integrated for testing reasons; it also spoke HL7v2, for our sins.
I may be in a position to open some of that up before it's lost.
I think the only other useful thing that wasn't added to the Foundation was some HL7 integration code that would sit in a customer environment and forward on requests to that backend securely. I believe that moved on with the software that it was primarily developed for, though, which is an in-hospital tablet based system for recording patient observations.