I had a beemer that exhibited the same brake judder issue even after changing discs and pads. Turned out to be the front tension rod bushes. I had already checked them and couldn't find any movement anywhere in the suspension. Checked it again with the car on shaker plates and the bushes showed up right away. The problem with jacking a car up to check for wear is that you can take some components out of their normal running position, which masks any play/wear. If you cant find a friendly garage with hydraulic shaker plates, get a mate to try rolling the front wheels back and forward with the handbrake on while you watch the bushes. Used to do that to check on old fords, problem is the Z is a bit lower so wont be easy to check unless you have a pit.