You can have Phoenix serve API endpoints and not include the phoenix html template package in your mix files.
> I’d prefer a simple React, Postgres, Elixir stack (REP).
That's not even a web stack. Web stack for example LAMP include a web server (Apache) or something that is able to response to client request. Elixir is a language it can't serve anything. You need cowboy or something and then build around it. Or you can just use Phoenix.