Not all at once. A ton at a time, for a long time. That was just an illustration of magnitude. Also, you have to mine and refine the rocks, so that the pure

Not all at once. A ton at a time, for a long time. That was just an illustration of magnitude. Also, you have to mine and refine the rocks, so that the pure metals can be turned into hydrides. Ceres wouldn't have enough hydrogen just as itself. It could still work as olivine to carbonate weathering. Cut Ceres up into pea-sized gravel, and drop it onto Venus, then use a solar shade to cool off Venus enough that metal carbonates don't immediately calcine into metal oxides. The exposed rock would suck up some of that CO2. No ocean, though.