I'm very hesitant to use either the words "leading" or "standard" with respect to HL7. HL7 is an incredibly backwards and antiquated model. It's a disaster to try and work with or implement in any way.
And, to make matters worse, it's not even really a standard - at least not in the way web developers would use the word. You have a separate HL7/ADT feed for every combination of (hospital, vendor) because no two work exactly the same way.
FHIR, which is what Apple is using, is its own standard, (created/managed through the HL7 organization), and addresses most of your concerns (https://www.hl7.org/fhir/)
I'm being sarcastic when I say HL7 isn't a leader. Obviously their psuedo-standards are widespread; they're just terrible.
I haven't dug much into FHIR because, when I last needed to implement any of this, literally nobody was actually using it. That said, based on everything else I've seen, I'm skeptical that it's actually a proper standard in the strict sense. Even the other message types that HL7 has produced are not properly specified, with ambiguous language that allows for multiple interpretations and "valid" (but incompatible) implementations.