My understanding is that it was water that started the whole thing. water vapor is about ten times as effective a greenhouse gas as co2. and the runaway heat started with too much water vapor(closer to to the sun I think), leading to more heat, leading to more water vapor. eventually it got hot enough to start liberating co2 out of the rocks. leading to the current situation today where venus has way too much atmosphere.

Which is to say, putting a ring around venus to block the sun may have merit, but adding more water sounds like pouring petrol on a fire.