For instance, if there is the content-type application/vnd.api+json, we need to register a custom JSON decoder, whereas the content-type suffix +json denotes that it's a valid JSON type.

This should be fairly straightforward - we could leverage https://gitlab.com/jamietanna/content-negotiation-go (a library I wrote) to perform the logic for spec-compliant parsing, or we could simply check if there's a suffix present, and try and match it up.