Thanks for those first comments.
I agree with your statments and I am already considering changing the round chafts for Hiwin type rail. Would the type HGR15 be sufficient or should I move straight to larger type ? (don't plan to mill hard steel)

The choice of the position of the y axis above the guides was dictated by the attempt to reduce dust pickup by the screw.ball
For the X axis, I had in mind the possibility to mount 2 screwball nuts to be able to reduce backlash. Currently only one is shown on the drawing.
However, I'll try to move the ballscrew upward to reduce the arm length.

Thanks