Well must say That's an ambitious first machine.?

Here's what I can add with experience of my machine being vertical.!!

The bottom of the gantry(Were turquoise plate is) will fill with chips if the end is closed so make it open. Also Chips will also collect on the rails so think about some form of shielding to deflect them or build in wipers.

If possible then I'd try to raise it further off the ground and tilt 15deg or so to aid work holding. Being so low it's a pain fastening material lower down also when you need to access material for say flicking small offcuts out the way etc.

With the Gantry raising and lowering in place of a typical Z axis then I'd connect the screws with timing belt rather than using twin motors, you'll keep screw sync much better. This setup will give good strength but will be so much harder to setup regards tramming the spindle. If there's any and by that I mean even the slightest 0.001mm difference in sync between screws you'll get steps when surfacing material.
For this reason I wouldn't use slaved steppers and tie together with belts or use Servo's with encoders and program drives with tiny error values.

Other than these things then it looks Ok but must say again it's very ambitious for a first machine.!!

Oh and why are those screws on the bed so far out.?