[cyberax]

Sabatier reaction is an equilibrium reaction, so it can only remove a part of carbon. You need to somehow continuously remove the generated carbon to keep the reaction from transforming it back into CO2.

The second issue is sourcing H2, water ice is common, you "just" need to redirect enough comets. Comets can be also used to build a solar shade, by placing them into a polar orbit around Venus inside the Roche limit. They'll naturally fall apart and form a cloud around the planet.