I don't know. Maybe if we compare our setups it might shed some light.
I have a warthog with no extension, no curves and a bit of fancy grease in the base to reduce static friction.
I give it a small roll input and yeah it rolls fast but when I return to centre it.... stops rolling as you'd expect.
It looks just like it does from camera shots in the real thing.
As for the inertia issue well. It is an extremely narrow aircraft with small wings and large ailerons I would say that a fast, almost inertia-less feel to the roll is to be expected?
The "nodding" I am experiencing but that's an issue to be solved by making correctly coordinated turns. With finesse it flies, in my view, quite realistically.
Of course I am not a real MiG-21 pilot, I've never even seen one fly in real life so I may be uninformed but I'm not experiencing the problem.