So, the author of Mark has his fictional account take place a few decades prior to his writing of the story. That makes it an historical account? Brilliant.
You're right. Its far more plausible that Mark invented a new genre, religious historical fiction, which was then believed by people all around, who never bothered to ask around to see whether or not it was based on anything historical.
The only thing which accounts for the the many texts which testify to a following of Jesus by people near to his life (both chronologically and geographically) is that he was a historical person who inspired a following, and mythic aspects of the story grew around historical accounts and teachings.
Your explanation of the story requires not only an entirely new genre of literature, which then died out after the gospels, it also requires a religious tradition entirely different from any other. You compare the "godman" Jesus to other gods of pagan traditions, but none of those pagans believed that their "gods" lived only a few decades ago. What kept their myths believable was that they occured in "olden times" long ago, whereas Jesus story was being told when people alive could contradict it.