Jesus' B-day was in March or April @ 4 years before he was born (4BC)
The early christians co-opted the Roman winter solstice festival called the Saturnilia. After all, who doesn't want a holiday at that time of year.
The dying god stories go back long before that (you left out Osiris). Read Sir James Frazier "The Golden Bough" for an
exhaustive and scholarly work on the subject (Yeah, Right!)
Just because other cultures have similar stories doesn't necessarily make them true, it just shows that humans have a need for something "Greater" than themselves to help to explain the world around them. The majority may rule, but it isn't always right.
By the way, "The Epic of Gilgamesh" predates the story of Noah but doesn't make the story of a universal flood any more real.
john