As Chris has stated, dropping your belly pan as a brake will cause your chassis to lose most of it’s torsional rigidity. One good hit and it will become a parallelogram. You are better off pushing individual pads against the carpet. Several years ago, one of the local teams did this. The pads were about 3 " x 3". They glued the wheel tread material to them. Two adults could not push their robot an inch once they were down.
Omitting the pockets in the gusset plates at the corners will allow them to resist hard hits better.
Do you have a separate picture of the “end caps”? They seem to be integral with the gusset on top and have a flanges that attach to the two plates running from to back on each side. It is difficult to see what they really look like but if they are the way I think they are, it will be very difficult to make the bends.
The plates that the gearboxes are mounted to have the bottom flange removed. There is a lot of mass (motor and gearbox) mounted to the side plate. In a collision, there will be a bending moment that can bend the side plate.
Place real components on your belly pan to check the size of the pocketing. The square holes look like they may be too big, forcing you to space the components out too much or making it such that the electrical components are attached to only one of the webs. More surface area will be needed if Velcro will be used to attach the electronics.
It may be advantageous to omit the holes in the top of the front and rear tubes to allow material so that the upper structure can be attached securely.
The tube across the middle will add more rigidity if it is attached to a plate at each end and those plates attached to the gearboxes in two locations.
Where is the battery mounted and how is it retained?
Is the gearbox easily removable for servicing/repair? Are the bolts that attach the gearboxes easily accessible?