It’s unfortunate that SAG called these “JSON Document Type” because they are not fully conceptually the same as document types. In the documentation:
JSON document type can be created only from a JSON schema…
These are a hybrid object, with characteristics of a document type and a schema. Many limitations associated with them. My take is to use them for validation only (if/when that is needed) and define a peer doc type that is used in the usual way that doc types are used.
As I’ve done elsewhere, I once again advocate for not performing payload validation “in the middle”. That aspect should be the domain of the end-points whenever possible.