The fact that certain myths and symbols predate Christianity (or Judaism) is an interesting but oversimplified way to look at the question of historicity.
I believe you have misunderstood my argument. When I said "the astrotheological elements are already present in the earliest documents," I am talking strictly about the New Testament, not writings from other cultures.
In other words, I would like you find the earliest New Testament manuscripts you possibly can, and you will find that all of the astrotheological symbolism is already in those documents. So claiming that this symbolism developed over time in Christianity is actually the argument that is speculative and without evidence, because there are literally no early manuscripts where the astrotheological elements are not already present. Those elements are actually central to the gospel narrative. The gospel would not be the same without them.