Great build (I've been following on Instagram)

I agree with your Z axis comments (moving carriages)
People often completely forget about the tool and too stick out and the need to get the tool over the top of the workpiece.

When analysing the Z axis only, moving rails Z axis may be slightly stiffer when the Z axis is fully raised, but only when the Z is fully raised (i.e. almost never) and to get the same workpiece clearance, requires a higher gantry which means less stiffness, all the time.