It’s not that there is one part of Mark where we can say, “Oh yes, Jesus really said that specific thing.” It’s Mark presents what is called a “low Christology”. That is, Jesus was a guy, who was especially wise and holy, and therefore rose up and become somehow more than a man and imbued with the supernatural. This is what his followers almost certainly believed about him during his life and shortly thereafter.

It is possible the James community thought that way but Paul certainly didn't. Also worth mentioning that Mark borrows the Latin fiction trope of the empty tomb meaning ascending to godhead. So even if Mark downplays Jesus while he was alive he makes him a god on his death.

Later gospels, especially John, present a “high Christology”: Jesus was with God on high and descended to earth and enlighten humanity, then went back to God.

Paul said nearly the same thing about a century earlier. What John added was the whole bit about the word. Paul does not talk about a normal person he talks about a celestial being who came to earth, did stuff, and unlike other humans (Paul didn't believe in an afterlife) came back to life in heavenly body form. Which meant he was the new Adam.

It’s been awhile since I’ve read the Gospels but I believe that there are some things Jesus says in Mark, like, “Don’t preach to the Greeks”, or “Avoid the Sumerians” that are right on the money, so to speak, about what a Jew form Judea at the time would have said. These statements are generally ignored by the modern Church because they contradict Christianity’s catholic/universal current state.

I am being honest and not snarky at all here but I have no idea what you are talking about. I did double check this morning and saw nothing like this in Mark. Could you quote the passage?