Hello,
Looking at the design the main problem you will have is keeping the frame ridged over these distances.
My take on how to improve it.. Use corexy on the moving frame to minimize the moving mass. Also, use 4 lift screws to raise it. The Duet can use a BL-Touch to level the frame with the stationary bed using the 4 steppers to minimize twist. If you use this approach then you also only need 2 bearings to keep the moving frame square to the stationary frame rather than the 4 shown. Less likely to bind because it is not over constrained. The two in opposite corners would be ideal.
Depending on your budget, linear rails will be more accurate and the interior space can be enclosed and heated.
FWIW.
Looks like a cool project. Make dinosaur bones!
Cheers,
Mark Napier