My “tall boy” was here:
The basic idea was that holding the top of the Z axis would support torque.
Some of the earlier thoughts were here:
A couple points of your analysis that stand out to me:
“Torque = Force x Lever Arm”
and
“The standard MPCNC handles about X comfortably”
Remember that deflection is torque * lever arm too, so for a given load at the tool, the deflection is quadratic with lever arm length, and it adds up very fast. And how much is “comfortable” depends a lot on what it is you are doing. PCBs with thin traces are unforgiving. Chasing tenths or hundredths of mm starts to get messy with all the things that can have an effect and you’re in uncharted territory pretty quick.
I have also gone down the path of: “if I keep the loads light enough by taking small enough bites, I can do anything.” Basically, if I am willing to go slower, maybe I can trade time for capability. Specifically, why is steel so difficult? Can I just (in spirit) use sandpaper for hours and hours? And the outcome was, I’ll just say it’s not that simple.
I don’t want to discourage you but just be aware of the likely issues so you’re not disappointed. If you go in with the goal to learn, you will always be successful.