Martin,
The usefulness of flywheels is dependent on the square of their rotational speed multiplied by their moment of inertia (which is related to their mass, and how far from the axis it is distributed). The very time at which you will want them to exert the maximum effect is usually when they’re going slowest, so have the least capability to do so. That said, they will reduce sudden accelerations, and sudden decelerations.
In model terms there is no benefit in removing mass near the axle, there is benefit in increasing flywheel diameter and length, and density of material used. In the full size, flywheels tend to have spokes because they would be unsupportably heavy if solid, or perhaps, for a given weight the wheel is more effective if that weight is located as far from the axle as practicable.
My view is that a keep-alive capacitor will do far more to help a model loco behave realistically than a flywheel, but only if you use DCC, of course. Easier to package too.
hth
Simon
