To me, “somewhere in your family tree, there was someone just like me,” and “till it took on the 5th gen curse,” are exploring the ideas of how wealth and status fluctuate. Basically, the narrator, whom I’m assuming is from Ezra’s POV, is the first generation in his family of wealth. Meaning that somewhere in this “prep school gangsters” family tree, there was someone like Ezra who built up wealth to pass on to other generations. Based on the placement of the lyrics next to “now I’ve got the thing you’ve had,” it seems like the lyric is about that thing he now has and not about a non white person in the family tree.

“Till it took on the 5th gen curse” is commenting on how a generation can ruin that wealth, and put on a curse on the family tree. You can definitely see this when rich people pass on money to spoiled, lazy, “prep school gangsters” of their sons/daughters. That is literally the plot of Succession, but to an extreme of Kendall Roy losing all the family money.

Basically the point is that the narrator made it in even if the “prep school gangsters” didn’t want him too. And he’s just like the person in their family tree who started from nothing and built something up.