"... then why do ancient religions pre-dating Christianity have stories of miracles that are the same basic story line as Christianity?"
This is not entirely true. Zeitgeist puts an anachronistic spin on
older religions, and sometimes just gets things wrong. Here are a couple of articles to consider:
"Why is Christ depicted with the Sun behind his head? Why do some church crosses have the Sun, or abbreviated version of the Zodiac?"
The bible doesn't depict Jesus at all. Drawings of Jesus come from imaginations of people and are not considered scripture. It's not surprising that peoples imaginations would be syncretistic at times, but that is more representative of artists than Christianity.
"Why is Sirius lined up with Orion's Belt on precisely December 25th, pointing at the horizon where the Sun rises on that day?"
The Bible doesn't put any significance on December 25th. Some people wanted to change the emphasis of a pagan holiday by celebrating something non-pagan on that day. But that doesn't relate to the truth or falsity of Christianity.
"Why is Easter celebrated at that particular time of year?"
Number 9:1-4 places 'passover' on a particular day in the Hebrew calender. John 19:14-18 places the crucifixion of Jesus on a particular day in relation to the passover, which has a particular time relation to his resurrection. Christians originally used the Jewish observance of the passover to determine when they should celebrate the resurrection of Jesus, but a few hundred years later other Christians decided to go with a computational method based on a different calender system.