Tell us about yourself!
Very impressive. I really want to build one of these and may use your design as a basis. I might try designing it with I-beams made from plywood instead of just plywood. It will increase the stiffness while probably making it lighter. Did you glue your wood together or just screw it? Glue and a little corner bracing will help with stiffness. Just a thought.
I still think the issue with stiffness is related to the lack of glue and the joints as one other user commented on above. Screws are fine for the prototype, but you'll need to improve upon those joints when you begin to harden the design and deal with structural issues such as stiffness.
Homebuilt (DIY) CNC router - Arduino based (GRBL)