I realized that my Z-Axis Stepper is not perfectly aligned with the threaded rod. This puts unnecessary stress and force on the Z-Axis Coupler. To get it aligned with the threaded rod I designed this simple 1mm spacer which is installed inbetween the stock mount for the stepper motor and the frame.
When they are perfectly aligned you can use a non-flexible coupler like me.
I added an 1.5mm and 2mm spacer too. For me 1mm was perfect. Since it is a very fast print just try it out!