Hi Paul,

Looks good thou got a Few things you may want to consider.?

I wouldn't have the ballscrew and motor moving with the table your just moving weight around unnecessarily which will limit the acceleration and seen has you want to mainly do 3D machining then acceleration is important. Given the bed will be heavy anyway this saving will become welcome.
This brings me to the next point.? The bed.!! It's a bit weak with no supports in the middle so I'd beef it up a bit, has you have it even if you used Ali plate for the Bed it will flex when plunging in the middle. A few cross braces will sort it.

Next the belt drive.? I wouldn't use those Idlers like that and just make the motor brackets slotted so the tension can be taken up by sliding the motors, KISS approach it works and it's easy.

Next Rails..!! If you haven't already got them I would use 20mm rails all round.? Reason being 15mm are very low with tiny bearings and on the Z axis they make it so you have to mount on quite high spacers to create the clearance for the ballscrew. Even with 20mm you have to machine in rebates for the screw blocks to clear.

Hope this helps.!