Couple of things, here. The 'story of Jesus's birth, if taken as false, is not a de facto declaration of a 'normal' storyline, ie. Mary is a Jewish woman, unwed, etc etc. The entire narrative could be inference as fictional, if you choose to disbelieve the virgin birth. My issue is, not your disbelief, that's fine with me, but the following expectation that people are supposed to believe the story at all. Why believe that the story isn't all made up, you already stated that the Bible has false, and true things in it. You are using a pre-set narrative, imo, fitting the Scripture into the 'story', that many secularists, and some other religious people as well, think that they are reading, when they read the Bible.
So, is the ''best guess', that Mary gave birth to a person named Jesus,, in Israel, in a barn, and that person became the quasi Rabbi fisherman carpenter whose verbal teachings were written down later by people who never knew Him, but they knew His teachings, and all the ''mystical'' stuff, (which is like, what, most of the narrative), is fictional? How does that ,make the Bible more credible? Or are you saying the Bible has no credibility?