Got a little more done on the frame, havent had as much time as I would like in the last few days. My bed is 16 inch on center and I decided to brace in between each of the bed cross members only the first ones in so far.
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Ballscrew
I need to finalize my plans for the X axis ballscrews so I can make mounts for them. Then motors mounts.
So a couple questions -
Do the numbers for the 2525 screw look good and make sense? On this long screw should I go with fixed:fixed mounts, I have read that this can stiffen things up a bit for a long screw.
Like some of the big guys do, can I get away with intermediate supports along the ballscrew and drive the screw? Or go the rotating ballnut route?
Do I even need the belt ratio on the X axis ballscrew? Would the motor speed be to slow without the gearing (~1600rpm at 1500ipm)? If I spin the screw, I could go direct drive with a 0 backlash coupling. Not that belting is all that difficult to implement it just takes some complexity out if I go direct coupled.

Motors
Ive run my inertia numbers multiple times with a few different numbers to try to account for some unknowns.
What I ended up with is, 2 of the 1.8kw motors for the x, the inertia ratios in my calculations worked out between 1.75 ---> 3.4 depending on the numbers. And the torque worked out between 4.25 -->5 Nm per motor, these motors are 6Nm so I feel like this is a pretty good choice.

Then for the Y I went with the 0.75kW motor 2.4Nm motor, inertia ratio calculations worked out between 2.25 --> 3.5 and the required torque was around 1.5Nm so a little headroom on torque.

I also picked that same 0.75kW motor for the Z, my inertia ratios get closer to 1.5 here and if I find issues I could change the gear ratio

If anyone has thoughts on these options for the motors Im all ears.

One thing, that I cant yet confirm, but it looks like the drives from Fred show a limit on the inertia ratio of 5. This was based on data I found online from who I think is the supplier of the drives.

Resonance
I think I will cross this bridge if/when I need to. In my day job I am a noise and vibe engineer (although automotive and nothing to do with machine design) so I may be able to figure out the correct countermeasures. I will say, as the frame sits it rings when hit, so I may borrow some accelerometers and do some impact testing (frequency response...) on the machine and just see where things are if I get ambitious.

Thanks for all the help this far!!!