It seems to me that biology is the best means of terraforming Venus. While Mars is more amenable to engineering as it is now, Venus might be made habitable by carefully designed organisms that might disrupt the planet's current weather and geology.

The first might be microorganisms that consume sulpheric acid and convert it to water and some sulphur byproducts. They could multiply wildly in the clouds, causing furious rainstorms and clearing out clouds -- which would recycle as clouds because of surface temperature, but with less and less of the sulphur. The process of clearing the clouds and cooling the surface might take a hundred thousand years -- but so what? The "germs" would probably adapt, might be supplied with genes that can be useful under changed conditions, but we will also continue to speed up the bio development from outside.

With enough information about Venusian conditions some rogue group might even design the organisms and small craft to transport them there outside government approval to start the process, which might become pretty quickly detectable. It would probably be impossible to stop once under way. Indeed, this is a goal for some here to begin pondering....