Russia has no interest in destroying the Ukrainian forces, because the latter is already a spent force.

For them, the only means of destroying NATO is when Europe increases their defense budget spending and leading to the crumbling of their own economy. This is already happening, and ending the war now gives room for Europe to breathe and rebuild their economy.

As you might have noticed, the militarization of European NATO states will paradoxically lead to the demilitarization of NATO instead.