Maybe to reduce or avoid the need for a separate thrust bearing mount you could insert the thrust bearing between the flex coupler and the motor housing ?
That idea also leads me to wonder about how high-speed running & resulting dynamic loads might affect the z-axis "helical spring" style flex couplers -
do they have any compressive flex to them, or are they so rigid that it's insignificant?
It also makes me wonder if the flex coupling is really necessary, considering that the top end of the leadscrew is free-floating;
It seems possible that the motor mounting plate might have enough flex to it to prevent any slight misalignment from causing harm to the motor bearings-
especially if that plate is made of plastic.
Of course its also possible the flex couplings are dirt cheap too so its pointless to try to avoid them