That makes more sense now in terms of the final design. Should have guessed with the O'Neil logo on the wall.

I think 10mm motor plate would also be OK stiffness-wise. What you will need to watch out for is if the bolt heads hit the belt then they will also need to be recessed. M8 cap heads would need 8mm depth of pocket so 10mm plate is then a bit thin.

In terms of recess or pockets for the motor - if you can find the drawings for the motors you might use you can check the length of the shaft. They are not usually that long plus you need access to the grub screw at the base of the pulley so that often means a pocket is required.

The motor plate extends past the extrusion and over the ballscrew - is that bit required? Are you hoping to bolt it to the bearing block as well? If not, you could cut it short and give more space for the pulley on the ballscrew. Just some thoughts . . .