I am not an expert, but I think you should wire the motors in bipolar series, not unipolar for better performance with that driver.
It might be good to post this question in the wood router build section. The reason I mention it, is that your screw drives are not optimal for speed either, and the gantry uprights might need beefing up.
60 ipm x 10 rotations per inch = 600 rpm - that is a lot for a 1/2 in dia. screw. For some reason, I am thinking that 50 - 100 rpm is closer to where it needs to be for better speed.
Edit - I looked at your weebly web site - very nice carving work. The fact that the lines are coming out so clean means that you certainly are doing something right.