Granite horizontal 5 axis build inspired by Grob/Kern

Hello all!
I have been spending way too much time rabbit holing on a new machine build and I wanted to share my progress and get some design feedback.

Inspired by @Fox3d and Dan Gelbart designs, I have been accumulating granite surface plates for a while, including one huge 4'x6'x10" beast ($100!!!) which I have made the base of this design. I want to get into 5 axis work and am very interested in the Grob and Kern machines. The design goals for the build are having a highly accurate machine that can have a decent 5 axis workspace I can use for R+D and small runs(I work in the robotics industry designing and building autonomous robots). Mainly this is a learning project.
Experience-wise in this realm, 10 years or so ago I retrofitted a Maho with Linux CNC, so this is not entirely my first rodeo, but it is my first new build of a CNC machine with modern drive systems.

I have been accumulating ballscrews, servo motors, robot parts off of ebay etc for a long while, so I have much of the drive and structure components already in hand.

The plan for the axes carriages are building them with 2"(or 50mm) Aluminum Cast Tool and Jig plate (eg Mic6 etc) as they come already precision ground, and then sanding them on the surface plates (like fox) to final shape. Anyone have a sense for how parallel and flat they are from the factory? I have only used material like this for making molds in the past.
The Kern machines use aluminum for their precision axes because it is good at dissipating heat quickly, and very predictable under thermal change, so there are no temperature gradients in the material. You keep the plates at a constant temp by running chilled fluid through them, and the kerns cycle a huge amount of the ethylene glycol per hour to do this. I bought an old SMC chiller, but not quite as big as what they are doing.

The spindle is a Fanuc new old stock spare from an old Hitachi Seiki HG500, which is huge. It powers a direct drive hsk63 cartridge.
The axis motors are Fanuc pulls from robots. m9 motors with 65k / rev absolute encoders and brakes.
Servo drives are Kollmorgen ethercat units.
The 5th axis table will have two of the large harmonic gears from an old Kuka robot I have (kr350), and may be an adventure in aluminum casting... We shall see!

Here is the current state of affairs. It is still plenty rough of course.

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Please give me any design feedback you have! I need to get this into FEA asap and take a look at how those plates are going to take deflection like @petereng is doing on his awesome milli project.