While I am not a specialist here, I have similar constraints for a project I’m working on, and I think the best path forward is to define a behaviour for the http calls and the json parsing, then it’s just a matter of implementing the callbacks for whatever library.
\Edit
You can then call functions from modules implementing the behaviour having said module declared eg in app env.
Benefits are : users can implement their own modules with libraries you didn’t think of at first and you can mock with mox.

Look how ExAws does it.

Behavior: ex_aws/lib/ex_aws/request/http_client.ex at main · ex-aws/ex_aws · GitHub

This is how we implement it in our project (using Mojito instead of the default client):

defmodule DB.S3.HttpClient do
  @moduledoc false
  @behaviour ExAws.Request.HttpClient
  def request(method, url, body, headers, http_opts \\ []) do
    case Mojito.request(method, url, headers, body, http_opts) do
      {:ok, response} -> {:ok, response}
      {:error, error} -> {:error, %{reason: error}}
    end
  end
end

The implementation module is passed through config. And used in ExAws like this: ex_aws/lib/ex_aws/request.ex at main · ex-aws/ex_aws · GitHub

And I think that such a config is perfectly fine for Json and HttpClient. I don’t see any usecase where you would want to have 2 different parsers in the app, right?

I’ve never built it myself in Elixir, but you can look at other libs for inspiration. Look how ex_aws does it with json GitHub - ex-aws/ex_aws: A flexible, easy to use set of clients AWS APIs for Elixir · GitHub

Maybe @benwilson512 could give some tips how to structure such a pluggable library. He mentioned some ideas in the ExAws thread. I would love a discussion about a a good architecture for a library