Venus isn't a better option. Mars is ready for human shelters on day one, venus would crush and melt anything we're sending there for hundreds of years. It'd be far easier to warm Mars than cool Venus, as we're discovering on earth, warming is much easier to cause.

For mars, You could crash nitrogenous asteroids into the surface, make GHG production facilities that emit CFCs or HFCs, and hopefully soil carbon gets released in a runaway effect. Thermonuclear weapons over the pole is impractical but a fun idea.

For venus, it's so tough, you'd have to block solar radiation and then fix the massive amounts of carbon with co2 capture... maybe if we develop co2 capture for dirt cheap on earth, we could make sky cities with carbon fixers on venus, after a few hundred years it'd be colder...

Fun to think of, but ultimately we need to terraform earth towards cooling, as our current warming is too successful