The Y axis front plate looks like it is a long way from the front of the gantry, perhaps 20mm or so? This reduces the stiffness for no real reason. I would aim for a gap of 5mm or so depending on how accurate you think you can make everything / flatness of gantry section.

Also the X axis connection down to the ballnut looks a bit less stiff than it could be. Can it be optimised a bit more? Maybe raising the ballscrews a bit in Z and mounting the ballnut to the [I]underside of the X axis bearing plate (bolting in the Z direction instead of Y direction). I think I would use another spacer / packer piece under the bearing plate to avoid raising the ballnut too much and creating a problem attaching the bearings and steppers into the frame at each end. You may even be able to mount the ballscrews closer to the sides of the frame (in Y) which all helps.