So I kind of got an idea...

The idea here is to just squeeze the 3D printed part around the motor shaft. I don't have any tiny hose clamps so I tightly wrapped electrical tape around it instead. It slips at about 1.5 to 1.8 kg·cm whereas the motor itself is able to create 9 kg·cm. I guess I could just put a pin in it as well for the times when it needs full torque, but let this friction fit keep the backlash at a minimum when it doesn't.

The tricky thing about this is that the motor shaft is only 10 mm long, and I kind of need all of that to wrap something around it to clamp onto it. That means that the wheel either has to be 1 mm thick like this thing, or the wheel is attached beyond the end of the motor shaft, which seems like it could cause problems.