I'm not sure a geared stepper would be a better idea. Certainly the gear train would reduce the load on the stepper motor itself - but with the inertia of the flywheel, the gear train will have every ounce of play and backlash exercised every time the stepper starts and stops.
In fact, I have to wonder what is the purpose of the disc. If it is to smooth out the motion, then there is a fundamental conflict between that sort of motion and the inherent nature of a stepper motor - specifically, a stepper motor needs to function in a stop/start manner on each step in order to function correctly. You could add positional feedback to help with this, but that is an extra step in complexity and will need to be "tuned in", especially for a system with high inertia.
Without meaning to offend, what you have done here is select a solution and ask for assistance in finding the component that will do the job, rather than define the requirements and then ask for input on possible solutions. This tends to lead into (potentially long) discussions that wander back and forth trying to fit a round plug into a square hole. Don't feel embarrassed by this, we see it quite frequently here where the solution chosen is not optimal or has its own set of problems.
If you feel comfortable with sharing a little more detail about the requirements of the motion system, you might find some alternative ideas. Details such as how often there will be start and stop actions; how much travel will occur in each movement; acceleration/deceleration; the total mass being moved and how (Is this disc the only mass that is being moved - or is there something else?) ... and so on.
You can sometimes find ideas offered by people who may have had similar requirements for a past project - and not only would you then have access to some possibly more suitable solutions, but to the problem solving that went with them.
Just a suggestion.