> I am tempted to merge it with the HTTP model as well, add JSON tags to it.

If you add a property which shouldn't be public you have to separate them. For example in the domain model you have a user with email but do not want to expose the email.

Second argument: I usually create a simple struct for each method. For example I do not want, that they provide the id for post requests. I want to send the id on get requests so that they can send an update request with the id. I prefer only "non-null" arguments and as a bonus you get a clean (auto-generated) api documentation.