[Chris is me]

It's super great that you've decided to teach yourself CAD and have worked on original designs just a few weeks into it. Good for you, keep doing that.

That said, this design might need some work, and building a cantilevered octacanum module like this is a somewhat complex endeavor that is probably not the best first design project for someone new to this stuff.

The "double cantilever" of supporting a drive module cantilevering off of an already cantilevered shaft can put some weird and strong bending moments on the drive shaft that it may not be ready to support. Specifically when a robot is pushing you from the side (or to a lesser extent, when you're strafing) - there's a real danger in bending the shaft this way and you'll want to be really careful.

For "west coast" drop drive modules like this, I've always been more comfortable with a design that straddles the drive tube, rather than hanging off of it. You can add some low friction plastic blocks between the module and the frame so that if the module deflects under load, the forces are transmitted to the frame rather than the axle supporting the module, and even if the axle supporting the module takes some load, it will be on both sides of the tube very close to the side walls which is a lot better of a loading condition. The Vex drop modules do this for good reason; they are a good starting point for a design.

If you are going to face mount the piston then using it only to push the module down is the correct choice. If you couple the piston to the module but rigid mount it on one end, you put a bending moment on the piston, which is bad news bears and just unnecessary. You'll have to pivot mount the piston on both ends if you want it to provide the up-lifting force to hold the mecanum off the ground, which is a lot more of a pain than just throwing a torsion spring on there.

If you're using 1/8" plates for the module, I would just get rid of the lightening altogether. The pattern you have has a web that is too thin to do anything in the middle and it gets dangerously close to those bolt holes. The lightening saves you maybe like a quarter pound per module?

Can you make the mecanum wheel a dead axle wheel? This way you can use the axle as a structural member and you get some "free" rigidity.

Overall, a good start!