It looks like a great well thought out first build!! 3mm aluminum (grade dependendant) will hold up well in beetleweights but I wouldnt expect it to last more than an event, maybe two at a strech. Steel is heavy on beetles, a few bots use HARDOX steel wedges however titanium (grade5+) is a very common wedge material. HDPE wedges have seen some success in the beetle classes aswell. If the robot feels sluggish then, presuming those are 12v 1000rpm motors, running them on 3s or even 4s (with approproate escs) gives those motors a huge burst in power.
Wish you luck in scotland!!