- πΊπΈUnited States tr Cascadia
I understand the use case, and it is an important one, but that's not within the scope of the Mime Mail module.
Drupal mail does not work on a basis where you compose HTML then send it. Drupal mail works by defining mail "templates" with the body and the subject already set, then sending those templates by filling in the To: and a few other things.
What you are asking for is a way to create and preview these templates. There are projects like https://www.drupal.org/project/pet β which can help in that regard.
Mime Mail is solely for taking content and properly formatting it so that it may be sent as an HTML email message. This is not supported by Drupal core, so Mime Mail is filling in this missing functionality. Creating that template content and previewing it is not something that Mime Mail will ever do.