[TexasSP]

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.