In general what you've drawn looks like a pretty strong design, but it's let down by only having one ballscrew on (what you have defined as) the Y-axis. For the width of your machine you really should use two ballscrews, one on either side of the gantry, particularly if you want to cut metals. Currently when the Z-axis is at either side of the gantry (so the limits of the X-axis) and a force is applied parallel to Y, you will get significant deflection. If the bearing spacing on Y is increased, this will help but you would have to increase it a lot to make a significant difference, which is clearly undesirable as you would loose travel.

Since you need a second ballscrew I strongly advise getting RM1610 and selling the old one since you will get much better feed-rates with the higher pitch screw.

The Z-axis looks good. You could make the plate on the rear thinner, say 10mm, but it's probably not worth it if you're going to get all the parts cut from a 20mm sheet anyway.