Has Christianity been copying most of pagan mythology and then reworking to fit Christianity’s religion? [Spirituality & Religion]

I think it's more subtle and complex than that.

Constantine recognized the power of the message of "Iēsus Nazarēnus, Rēx Iūdaeōrum" to simultaneously unite and then therefore influence a lot of people to a common cause. He was a marketing expert ahead of his time. While promoting early Christianity, he himself waffled and waited until he was near death before becoming baptized. He didn't truly care about religion. He just knew that religion was the easiest way to lead.

He built St. Peter's Basilica and got the ball rolling.

Effectively, over the centuries, Christianity has been used by many world leaders to control the population, using the "word" as a rallying cry to unite. In order to unite, Christianity needed to evolve and become something meaningful.

- the birth of Jesus was likely not in a northern hemisphere winter, so December is very unlikely. But the early Romans tended to celebrate pagan festivals like Saturnalia which ends on Dec 23rd. This allowed for Dec 25th to be a convenient extension of the festival.
- The Celtic cross has a circle in it, which represents the sun. Ancient Celts were pagans who worshipped the sun.
- Haitian Catholics are full of hypocrisy. The Bible says that idolatry is bad, but yet Vodou in Haiti literally uses statues of saints in its rituals.

All of the Christian churches adopt local traditions, but none more than the Catholics. Most of the protestant sects evolved from a sense of, "we like where we are... we don't need to change anymore"