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I suppose ideally you would want a support for the leadscrew that wasn't through the motor shaft, like a motor coupling that took the weight on a bearing with thrust washers each side of it, like in the CNC world. But we're not dealing with particularly heavy loads, and it's not moving most of the time. The advantage with motors at the bottom is that the support the motor leadscrew solidly from underneath, though I haven't done this properly yet.
I've just realised that this thing does exactly that, sorry! It provides vertical support of the x-axis at the bottom, takes pressure off the motor bearing, as well as guiding the rod. Ideal! You could further improve it perhaps, by making it footed, to directly support the rod on the ground.