The man benefit of granite plates is that you can lap them yourself which is much cheaper than having to send casts to a company to machine surfaces for you.

The Z axis is the weakest point of this design, the lower it goes the less stiff it becomes. In a raising gantry, having the spindle mount directly on the bridge the stiffness is equal throughout its travel. Speed is not an issue because most of the machining is done on the XY plane, Z direction is also much stiffer. Some of us have done basic FEA on this design, check peteeng milli thread. A double column raising gantry scored the best in stiffness. If you have concerns about moving the bridge up and down you can upscale the ballscrew by a size, you have space for that because they would mount on the sides.

FM30 is overpriced in my opinion, for the same price you can get a SAB150 from olispeed. https://www.olispeed.com/electrospin...change-tool-2/ it packs a punch and at least the manufacturer doesn't hide the torque curves.