FHIR APIs are definitely becoming more and more broadly deployed, which is awesome! Scaling wise, it might be worth looking into the interoperability partners that specialize in standing up FHIR endpoints for providers like Datica & Redox.
Agree on FHIR APIs are becoming ubiquitous especially with Apple Health pushing its adoption in consumer facing applications, not so much on the legacy enterprise side from what I've seen thus far (I work primarily in the clinical trials side of things) and we prototyped our own integration stack to help with wrangling with different standards (more like the lack thereof). It is worth actually building that ourselves in the early days as we have learned so much about the challenges to integrate with our partners and will at some point looking into interoperability partners as we scale.