I have sought some means to keep the speed down and yet still store energy. I have thought of a tube with a piston pressing on a gas that when sufficient pressure is achieved that a phase change would store energy toward the rim as it pulled a vacuum on the axle side. Or vice versa. Thus at a given temperature the wheel would move the piston back and forth thus altering the rpms of the wheel.

I thought of using magnets in a similar manner. Mount a permanent in the outer end of a pipe and another that when at speed is centrifugally pushed toward the end one and is repelled as the two North ends face each other. The faster the wheel the nearer the magnets get and the more repulsion they feel. Or somehow use electro-magnets in a similar way.

The goal is to pump energy into the wheel other than kinetically. The main storage would be in a narrow range of rpms. Yeah that be the goal.
Just some thoughts.