Which of the two approaches to routing a module settings page is the best practice?
Without knowing what is in the controller this would be the second one. The extra custom controller class only makes sense if you do something in myMethod() the form controller invoked by _form is not able to do.

See https://www.drupal.org/node/2117411#fapi-in-route

This integrates the form in a request so that it can receive route parameters and the request itself as well as returning a response if necessary.