A new hope: Epoxy granite and steel.

[JAZZCNC]

I like your new machine, though I think hiding the ball screws like that reduces stiffness too much by cutting into the tubes.

Well that depends on application and what stiffness is required. In this application it doesn't need high stiffness but does require the height. However, I can tell you it's still very stiff.

I will be building a much larger version that will work the same with lifting gantry but with much more bracing using 10mm wall tubes rather than 5mm used on this.

You can have a single central screw on a lifting gantry machine. The screw moves up and down with the gantry. Nut it attached to the fixed top cross beam. Either rotating but with servo attached to fixed cross beam, or rotating screw with Z servo moving up and down with Z.
Pros: Single screw (screw mapping, servo tuning, cost). Cons: A bit trickier to implement. Will have screw sticking right up high when Z fully up.

You could but it will look crap and require you have a high ceiling.
Also it won't work very well because the gantry could and IMO will definately rack when plunge cutting at the outer edges. Disaster waiting to happen IMO.

The design I've shown only uses a single motor connected to the screws with belt/pulleys so it's only one extra screw so no big expense in the grand scheme.