The "IETF document as URN" RFC doesn't address sections/anchors in the document.
Should the full URN for https://datatracker.ietf.org/doc/html/rfc2648#section-3 look like urn:ietf:rfc:2648:3 or urn:ietf:rfc:2648:section-3? Is the urn:ietf:rfc:2648#section-3 notation expected to resolve correctly?
As you note yourself, the official definition for this namespace does not provide a way to reference sections within a document. That means there is currently no "right way" to do what you're trying to do. A standards-compliant URN parser ought to flag any of your proposed strings as invalid URNs, because they violate the specification that only digits are allowed after urn:ietf:rfc:.
To create a "right way" to do what you're trying to do, join the IETF and propose your extension to the namespace using the typical IETF procedures for drafting new standards proposals.
