You are absolutely right. I checked wikipedia ([1] and [2]), and it turns out that the mass of whole asteroid belt is only about 6 times as high as the mass of

You are absolutely right. I checked wikipedia ([1] and [2]), and it turns out that the mass of whole asteroid belt is only about 6 times as high as the mass of the Venusian atmosphere, and half of this mass is taken by the top 4-5 asteroids, which arguably we wouldn't want to start flinging around the solar system. Moreover, most of the asteroids are far from the Kirkwood gaps (almost by definition), so if you want to nudge them you would lose a lot of mass to ablation.