Want to cut aluminium? FWIW could I suggest a different way to make the machine?

Start with a good solid base, a couple of thicknesses of beech worktop sounds good. That's your anti-vibration inertial lump.

Put an aluminium plate, side to side, across the middle. About 8" wide sounds good. It's only a washer so it doesn't have to be too thick.

Bolt the gantry to the plate and base. Butress it. Get a good solid Y axis.

Bolt profile rail blocks facing upwards on the plate to hold 2 rails 8" apart. Screw the rails to the bottom of a 12 x 20 bed plate. You now have a good solid X axis. Your cutter will never be more than 4" from 2 carriage blocks in Y and 4 carriages in X.

Because the X bearings are only 8" apart you can use one screw up the middle, it isn't going to twist. Don't make life difficult trying to drive 2 motors together.

Make the sides of the gantry too tall so you can move the whole Y,Z assembly up when you come to do your 4th axis.
I'm expecting to be told I am out of my tiny mind, let's see what they can come up with