I think that there are several questions here that need to be separated. The first question is whether Jesus actually existed as an historical person. The second is whether Jesus was God. The third is whether the various accretions that have occurred in Christianity over the years are actually Christian in nature. A subset of the third question is whether something drawn into Christianity from the outside, like Christmas, can be "Christianized" or whether it is always tainted by its non-Christian religion.
This is a topic where we need to work forward. To say that non-Christian things have been drawn into Christianity tells us nothing about whether Christ existed in history or whether Christ was God. To attack the notion that Christ was God by saying that the various god like things about Him were derived from other religions says nothing about whether He existed in history as such.
The historical record of Christ's existence is contained in the gospels. Even if we discard the gospels of Matthew, Mark, Luke and John, we still have the post-resurrection books. To say that those are all fakes would imply that some sort of massive conspiracy existed in historical time perpetrated by people who would have been Christ's contemporaries. So the obvious question is not where is the extra biblical evidence that Christ existed, but where is the contemporary evidence that He did not.
As for extra biblical evidence about the existence of Christ, why should there be any? Christ wasn't Caesar. There were no monuments to Him in his lifetime. He ruled no nations. He won no wars. In history at the time, He was an insignificant person. A lack of an extra biblical record proves nothing. There is not, for example, any evidence in Egypt that Moses existed. This does not mean that Moses did not exist. On the face of things, given that Christ as a historical personage was a poor carpenter and a non-citizen of the Roman Empire, I would say that books of the New Testament provide an extremely rich historical record of him for the time. I don't think we know as much about any other common person of the era.