Chaz,
have not been here long, just looked at your initial design .

2 major flaws pop at once:
-the gantry profiles
-the bearing blocks for the Z

Let me explain how i see it and what i mean. I would compare with my design / the violet one/ so its easy to understand, as mine is overbuild also.
-If you take a closer look on the reference machine from the pdf you would see that these are not profiles, the whole gantry is massive cast and braced piece. meaning that your is flimsy in comparison. The only way i see it is make it like my machine- solder a additional reinforcing plates at the gantry beams to tighten all up. Thats why in fact i did not went with that design, cause while on Johnatans red machine it works, for a bigger machine gantry needs serious reinforcing. So i designed my gantry/a typical style/ so its more rigid than the gantry you have in mind. The ideal super machine for me would be combined between both designs. meaning 3 profiles for the Z left right. An overkill. Say no more than 200Euro more on rail and profile


-For the Z i have no doubt. Carriages must be fixed and rails must be on Z , for ultimate strength. plus better 2 rails x2 carriages than 4 carriages and 4 rails. The arm of force, the lever. Now its good to have in mind that for that type of machine and 2 rails with Hiwin 20 x 2 carriages is overkill. So no need for 4.
I bet that the way i designed it /30cm lever arm and 20cm z travel is more rigid than not properly designed Z with 4 carriages.


So why they made that original monster like that. Cause for sure the spindle is much bigger power and weight than what you will use, the gantry is cast and generally is made for hard metal, not aluminum.


I will be happy to explain better if i not clear what i say. Soon i will be able to prove what i am saying, when chips start flying from mine machine.