Couple of things:
- The sheet thickness is almost certainly fine. See example below. However,
- Using a router to cut material will probably be challenging. As I understand it, the bit will tend to grab the material and yank it around with thin sheet. Granted, I don't personally have experience with this, but I've heard this said previously in other threads
- The sides are probably beefier than they need to be. Again, see example below
This past year my team and Team 1983 used a similar sheet metal design:
FRC 2374
FRC 1983
Both of these designs were from before kickoff and thus modified to address the challenges of the game.
Both teams only used one piece of sheet metal on the side panels, not a full box like you have. We then pop riveted the sheets to pieces of angle aluminum. Our rivets were on a 2" spacing. The chassis seemed to hold up quite well.
Good work though, good luck on your future iterations.