I think you are on the right track. Just some of my thoughts. Impressive that someone is willing to weld the frame. 40x40x2 steel is overkill in my opinion. Could get away with half that as long as you have a solid base.
I would try to size the tube a little higher unless your 10mm ply has only a few plys.
I can’t imagine getting an Uno to work very well. Maybe use a better grbl processors or find a way to get a DSP controller. I never heard of anyone trying to program a laser controller. Interesting about the honeycomb, there are many other ways to go about holding the work flat.
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