Hello,
the fact that you admit that you did not believe that your students would be able to solve the task suggests to me that you are open to surprises. Or that they are not only able to solve certain maths tasks that they are "actually" not yet capable of because of your teaching alone. It's nice that you share something like that.

One question about your first paragraph. What alternatives do you see to the rewards or punishments mentioned? Is there something else? And if so, what would that be?

Greetings.