Reddit reviews PGN - 5/16" Inch (0.3125") Precision Chrome Steel Bearing Balls G25 (100 PCS)

Remove the constraint on the leadscrew at the top. That's helped me in the past. Also, your leadscrew can be straight as an arrow, but simultaneously be at an angle because of your coupler. Take everything off the leadscrew so that the only thing you have left is the motor, coupler and screw itself. Turn the motor and see if there's runout. If there is, then try to see if you can re-insert it and tighten up the set screw without pitching the leadscrew to one side. Lastly, a trick that might help you is to put a metal ball bearing in the coupler between the shaft and leadscrew. 5/16" is the perfect size to fit in on the leadscrew side of the coupler. You want the ball bearing to make contact with the motor shaft and screw. The benefit to doing this is that you retain the angular compensation of the alignment coupler, but get rid of all the springiness of it. If you have the screw and shaft touching without the ball bearing, then the screw actually has to lift up a little bit and stretch out the coupler when it's compensating. If you leave a space in there (which is what you're usually instructed to do) then you might find that you've got a hysteresis problem when doing Z hops because the coupler is stretching and compressing without returning to its normal state repeatably.