The 'Son of Man' makes his first appearance in Daniel 7:13-14, as 'a son of man' ie human not angelic, an earthly being whom God empowers, such that his rule will be everlasting. In 1 Enoch, written one or two centuries BCE, he moves from a son of man to the Son of Man, with a similar role. He's naturally resonant with the Apocalyptic mood in the Jewish world in NT times , because he'll overthrow Israel's conquerors and restore Jewish political autonomy ─ as a good messiah should. Just as Jesus didn't fit the character of a Jewish messiah (being neither a civil, military or religious leader of the Jews), nor does he fit the character of the Son of Man ─ but of course his adherents were happy to give him both titles. The question is whether he thought of himself as the Son of Man, and I don't see how a Jesus who advocates peace with Rome and the payment of Roman taxes and civil order and obedience fits that role. Of course, he's waiting for the Apocalypse, when the dirty work will be done for him by the Son of Man or alternatively by God.
The first gospel written is Mark, where (Mark 3:38 ) Jesus speaks of the Son of Man as someone other than himself. Check out Matthew 24:27. Notice Matthew 24:34-40, where the king (the Son of Man) never mentions belief in him/Jesus as relevant to salvation, or Luke 21:34 which again refers to the Son of Man as distinct from Jesus.
I find such things more persuasive than the occasions when Jesus is equated with the Son of Man ─ I think that's a later add-on by Jesus' followers.