We provide API integration solutions along the lines of other companies. We have not been around as long as Catalyze (which does a very good job) but are expert

We provide API integration solutions along the lines of other companies. We have not been around as long as Catalyze (which does a very good job) but are experts in FHIR, and have an infrastructure that may be more multi-party friendly. We have a pretty cool multi-tenant database solution that hospital parties could pool data into with highly granular authentication, access control, and auditability at both the db and API levels. No one else offers this yet, but it also may be more robust than the hospitals understand enough to care about. Happy to discuss if you are interested at the commercial site listed in my profile. Also happy just to share our experiences in similar situations, or make recommendations for other more simple solutions on your own stack. Good luck!