Looks great! A couple of suggestions, the black rails on the outside seem like they might be a little weak between bearings. I'd recommend maybe adding some triangle cross beams to increase the structural rigidity in those squareish holes on those rails.
In addition you might be able to take some more weight out of the belly pan with a more efficient lightening hole pattern such as alternating triangles. Squares are not the most rigid of structures so you'll be able to remove more material with another pattern.
To take it to the next level maybe add mounting holes for electronics into the pan, in addition, if you have the capability with your manufacturing process, I'd recommend adding some flanges in there, flanges will increase structural rigidity by a remarkable amount.
Looks great and best of luck continuing on your robot design journey!
