Laser cutting should work fine. That is one of the goals of the project. The motor mounts need to be stiff, so use a stiff material there. I used some counter bores on the Y carriage, that helps when I print on glass, but I also use a heater that I stand off on small spacers and that would not care about some screw heads. A simple solution is to manually countersink the holes and use flat head screws. The design is independent of the thickness of the carriages, so you might use 6mm like I did on my prototype. I think I have 1/8" now for my sheet metal quoting.
I am having a little difficulty with a single universal extruder mount. I have something similar to your idea designed, except the sides of that carriage fold forward and a flat plate bolts to the bottom of that. I am hoping the 2 plates can accommodate most of the extruders.
"If you didn't build it, you will never own it."