Good to see someone using the spreadsheet. You'll find that if you change to similar size box section the deflection due to bending will be similar, but the bending due to torsion will be substantially lower. That's because the torsional stiffness of a channel cross section is very poor. Also put the rails for the Z-axis on the plate that moves up and down, so swap them round. It's much stronger this way since the distance between the supporting bearings and the tool is minimised. The best thing you could do to that design is loose the tall gantry sides, as they're invariably weak compared to the machine frame.