Erm i think you should look at your Frame section size in the model because it looks too large to be 50mm and out of proportion. This can affect how things fit so worth putting write now.

As for the rest then it's mostly there with just a few exceptions.
You need ends on Gantry to help stiffen the gantry up.
Brace the ballnut Drop bracket on the Gantry ballscrew and try to keep the Drop brackets on the Slaved axis to shortest possible length.
Put some short legs on it or adjustable feet it will make setup easier.

1 x 3Nm motor driving two screws with belts will struggle so I'd think about using larger motor. 6nm would be better but unfortunatly this means nema 34 and plenty of volts. Idealy Mains voltage drives.
Or other option is to use higher pitch 20mm and gear it 2:1. This will double your torque and lower the actual ballscrew speed so reducing chance of whip and resonance but will give roughly the same speed as 10mm pitch.

Material thickness then 4 mm for steel box and if Gantry is Ali then thick has possible idealy 10mm.

Yes Air is required for cutting ali properly mainly to clear chips and stop re-cutting.