The term “a stone that God cannot lift” means “a stone that the omnipotent one cannot lift,” and that is like a triangle with five sides because it describes an existing state with a logical contradiction. But the question whether God can create a stone that He cannot lift is a question of whether in the future He can create such a stone — that is, limit Himself in the future.
And my real question is: is an omnipotent being who chooses, by free choice, to turn himself into a non-omnipotent being already a logical contradiction now?

I don’t know what “already now” means. A state in which the omnipotent is not omnipotent is contradictory. Can God create a round triangle tomorrow? What difference does it make that we’re talking about tomorrow?! God is no more omnipotent now than tomorrow. That same attribute of omnipotence is supposed to characterize Him at all times (what is special about this moment as opposed to tomorrow?). And if He can be non-omnipotent, then who told you that now He is omnipotent, or that He ever was? This is really a discussion devoid of meaning.

Right now He is omnipotent, and He also has the ability to make Himself limited if He chooses. Why does that detract from His attribute of omnipotence?

Thank you very much for the responses!
Even though the Rabbi thinks the discussion doesn’t lead anywhere.

Shimon, the potential to ever reach a state of inability — such potential contradicts the concept of omnipotence. (Already now, He cannot limit Himself and then change His mind about the limitation.) Therefore this is a logical contradiction.