One thing I have not seen mentioned is the stiffness of the thin table, given the 8x4' dimensions and the different expension coefficients (by temperature and humidity) vs. the steel frame. I think there is no way to make a thin slab that would not move a little with environmental changes. From all the earlier proposals I think most promising would be the granite (but hard to get in 4x8 and impossibly heavy), a thick and ground MIC-6 cast aluminum plate (really expensive) or the epoxy glass concrete (would probably need some surface grinding to be flat and be super heavy as well). At the end you may effectively be looking at building a new machine.
One low cost compromise may be to look for a sandwich torsion table design. That means lightweight rigid plates (e.g. good quality plywood) spaced apart (let's say 4 inches) by a grid of stringers. This hollow symmetrical sandwich could serve as vacuum plenum as well when sealed well. It would probably not be totally flat as constructed but the spoilboard surfacing could compensate for that. For that matter, if you glue a spoilboard on the top I would glue the same board to the bottom of the sandwich.
Now, I have no idea if that could be fitted to your machine and you may lose some z-travel. Just talking theoretically....
FWIW, my 3x4' machine has an 80/20 extrusion table. Works pretty well but it is just a bunch of extruded aluminum sticks. To make flat table that does not flex, I bolted them to a rigid sandwich table as mentioned above.
Digital Vacuum gauge
Not sure who is selling this, but for those who have to be “digital everything”
https://springfield.craigslist.org/t...228719866.html
The decimal point seems to be the most important on the z axis... x & y not so much....
ShopBot... Where even the scraps and things you mess up and throw away are cool....