For a CNC router it's not difficult to achieve +-0.05mm.

Your Y and Z axis designs look good.

I would increase the size of the triangular plates and add some to the other two sides. You could also use diagonal pieces of box section to reinforce the X-axis. If you can add a piece of box section across the back between the X-axis rails that will make it a lot stronger.

What thickness are the gantry side plates? I'd reccomend 15-20mm aluminium. The X-axis ballnut mounts will be much stronger if you extend them down and mount the ballnut directly to the gantry side. The ballnut mounts are particularly important since any deflection in the ballnut mount directly leads to tool deflection and inaccuracy. You could also add a triangular support between the X-axis bearing block mounts and the gantry sides to help stop that joint bending.

If you don't mind posting the solidworks model I can open it to have a better look.

What ballscrews are you intending to use? For that size I would reccomend RM1610 on X and Y and RM1605 for Z with timing belts and pulleys on all of them.