That's ambitious! An ATC spindle of your own design? Do you have experience with that? Please post how you do it and your calculations for figuring out the motor windings, torque vs speed curve, Amp draw, ATC system, all that good stuff please. If you know what you're doing I'd like to learn from you. If not, this aspect will fail miserably. Either way, it would be an interesting read.
What's an mu? Keep it simple mate, inches or mm. Did you do an FEA on this?
Did you also include dynamic deflections due to machine accelerations in your calculations?
16mm seems small to me for a machine that is good at milling steel. What are your goals for top speed and acceleration?
Pneumatic counterbalance using pneumatic cylinders or gas struts? I don't see it in your drawings.
Have you considered looking for some really flat big box square aluminum tubes? I say aluminum because in my limited experience it generally comes flatter and straighter than steel ones. I have to think it might be easier to make the structure out of square tube sections, then fill the entire thing with epoxy granite, leaving the more attractive and wear resistant aluminum as a permanent skin.