Augustus is known for and by his historical significance, and least of all for any myths that may have evolved about him.
So what? Jesus was famous not just for casting out demons and healing (acts which historical people in the 20th century have also been credited with). He was well known for his teachings, e.g. by Josephus.
The relevant point is that being credited with miracles and magic do not bar one from being historical.
Comparing him to a character of mythology with no secular bio
There is no such thing as a "secular bio" in the ancient world. In fact, there weren't even biographies as we would think of them. There were "lives" but none of them were "secular" as the idea of seperating religion or the religious from life wasn't really around. Hence, none of the lives of the philosophers, of the emperors, and so forth, are "secular." Of course, you haven't actually read any of them, so you wouldn't really know.
the mythological status of a miracle worker to that of historical
Except he is only "mythical" because of your nearly religious faith in this concept, as well as your nearly complete lack of familiarity with both relevant primary sources and secondary scholarship. Do you have any idea how many people in ancient history have far less data about them, are also credited as wonder-workers, and are considered historical?