Hi Dan (@iMakeRobots),
I think I have seen some pancake shaped stepper motors that have the entire stator and rotor accessible so the torque is distributed over a wider area than just a thin shaft; believe the motors were from an old VCR. Hard drives have servo motors that also have a “wide” moving surface.
This way there is no “concentrated” loading of the plastic at the bolt or key interfaces and getting shredded.
Use epoxy (or even velcro, velcro might “yield” some) to attach the moving arm surface to such motors.