It’s easy to check how flat your bed is with a straight edge and a flashlight. Put the straightedge on the bed, and shine a flashlight from behind. Any light you see seeping under the straightedge is unevenness.
People on the forums have spent some time manually correcting really bad beds. Two results I found searching the forums for “flatten”:
I’ve also heard that some people will preheat the bed, then quickly do a 5x5 calibration before the bed cools off too much. Although there was talk of the firmware preheating the bed before calibration rather than doing it cold. I haven’t kept up with that (I have an Original, so I only know about the 2.0 problems from reading the forum.)