Nearly slipped thru the net this one.!

Ok well first the base frame is pretty much ok, just a little bracing on the corner uprights would call it finished.

The Gantry needs a little work and to be honest, I think you are making it harder than it needs to be with this design. If it was me then I'd just stick with a conventional setup and stump up the extra cost of buying a piece of profile. Look for designs that use the "L" shape layout I came up with years ago, it's tried and tested on 100's of machines large and small. It also makes fastening rails and ball-screws much easier.

Next, the Z-axis is a complete right off.!!
I'm assuming you meant steel 5mm plate.? If so it just isn't strong enough and neither would it be flat enough. If going to use steel you'll need at least 10mm and even then it would need to be machined flat. So again keep it simple and go with a 15 or 20mm aluminum tooling plate, it costs a little more than steel but it's easy to work with and just gets the job done.

Lastly component selection.!
The X and Y ball-screws are the correct choice but I wouldn't go with 1204 for the Z-axis, either 1605 or 1610 would be my choice. 1610 allows you the freedom to connect the motor to the z-axis screw with a short HTD belt and apply a ratio, the belt drive helps with resonance and the ratio means you can tweak the Z-axis to do whatever you want and handle a heavy spindle if needed because the torque increases with a ratio.