Some good development and ideas, but where were you thinking of putting the Z axis leadscrew? Looks like there's no space now.

I wonder if you just turned the Z axis rails through 90deg and mounted the support rail onto the router plate, and mounted the blocks back onto the main Y axis carriage, you would be better off. Stiffness wise it must be similar, and you are no further away in terms of spacing from the gantry than the above dwg (judging by the dwg above). You could even include the angle pieces in there as well if you want.