Thanks for the feedback.
Both comments make a lot of sense so I will have a think about it and come back with a better design. I will change to 1610 ballscrew. Still just learning Solidworks so it might be a while. If going with the fixed gantry design how best to beef up the gantry? Triangulate, gusset or just larger box section? Also does a design like this really need 2 ballscrews on the z axis? Would it be ok to have one in the middle?