How do I get a textarea form field to use a full HTML WYSIWYG editor?
You want to use the TextFormat form element: https://api.drupal.org/api/drupal/core%21modules%21filter%21src%21Element%21TextFormat.php/class/TextFormat/8.4.x

Note that this will give you a value and a format in a nested form structure, you need to store both (or lock to a specific format) and then render the content with that format to avoid security issues.