[Answered ]-Want to make it obvious that I can reverse reference by related_name in Django

You can probably add an annotation:

class User(AbstractUser):
    posted_quizzes: QuerySet

Is there any way that properties added via related_name can be seen directly from the User class?

The professional edition of PyCharm (JetBrains) for example has an extra module to have more "understanding" about Django, it thus can inspect the Django models, and can automatically perform autocompletion.