Re: Z axis idea
An image search for 'plano milling machine' turns up machines of this type of configuration. Possibly because the design resembles a moving table planer ? https://en.wikipedia.org/wiki/Planer_(metalworking)
I worked in a shop with such a beast but it was cnc in xyz only, the y bridge was raised by large acme screws which were synchronized but not cnc controlled.
The bridge was seldom raised as the machine was primarily used to drill flat bar. Lots and lots of flat bar
Anyone who says "It only goes together one way" has no imagination.