Please Don't get me wrong on what I'm about to say has I'm not pulling down what you have designed, it looks and sure will work great. But This design seems an awful lot of work and complexity and extra cost for no major benifit over a fixed gantry design.!

By this I mean the wide Gantry and wasted space at each end mean it will have about the same foot print and give about the same cutting area of Fixed Gantry.
If used Fixed gantry design with lifting Y axis allowing none lifting Z axis meaning minimal Z axis extension, essentialy just tool length would have been much stronger and far simpler design IMO.

Like I say just an observation which for the sake of others thought I'd mention and not pulling down what you are building or doing. It's looking great sure will work great and i'm looking forward to seeing it come along.