1/8 or 3mm should provide the strength you need. Maybe up to 3/16 or 5mm on the motor mount (although I still think 1/8th would do it on the motor mount too). The support braces would probably need to be 3/16 or 5mm for sure to allow for screw holes.
One more note. I would not try laser cutting the screw holes, it will turn out messy as aluminum creates to much slag on the laser even if the setup is perfect. The screw holes should be done on a CNC Milling machine.
everything is cut from 3 and 2mm 7075 alu, the motor/diffmount is made from 7mm alu.
it's nearly finished now, I only have to bend the front and back kickup and make some spacers.
then I can finish it (polisch it).
Have you test fit the bulkheads in it yet? How did you measure the hole locations for the bulkheads? (I'd be interested in obtaining that data from you, if possible).
If it were me, I'd C'sink as many of those fasteners as possible.
btw, looks great!
Losi 8T 1.0, Savage Flux - XL style, LST XXL, Muggy, 3.3 E-Revo Conversion and sitting outside 425hp, 831 Tq Dodge Ram Turbo Diesel. It SMOKES