If your goal is to mill harder materials, I would stay away from that round rail, and use profile rail. It's about 10x stronger than round rail for the given size series, and can receive the same load in all four directions. Unlike round rail, whose load rating depends on its orientation.
You may have not drawn it in yet, but some triangulation I feel is necessary, especially wit the heavier workpieces on the machine. I'd also consider using at least 32mm or 35mm screw on the X to prevent sag. Some way to protect your ballscrews and LM guides from marble dust/slurry.