Good post - I agree with nearly everything you said. I’d suggest that since to actually perform a conformance test, at least one (or some particular mixture of) concrete serial formats are implicated, that the serial formats are one dimension of a ‘technology profile’.
For example, vendor A’s system might run on Oracle only, and support JSON and XML.
Vendor B might run on any mainstream RDBMS, but only support XML.
Vendor C might only run over Linux/MongoDB/etc, and support JSON.
Vendor D - .Net, …
and so on.
From a procurement perspective, the particular technology infrastructure usually includes things like OS, DB, virtualisation / container management, and arguably, serial formats.
I’m sure this is not the only way to analyse this…