This turned out really good, planarity actually *improved* as I got deeper which surprised me because usually it's the opposite. Not sure if I was getting the feel of the part and correcting as I saw unevenness, or something about the way it was flexing, or what. But it seems like there is less edge rounding now.
There's a few obvious thicker zone fills, maybe power tracks? And a lot of signal routes, including some length-matched diff pairs.
Most of this layer is one big zone fill that I suspect is ground but have not probed to confirm.
