> we'll need to think in terms of millennia, not lifetimes

I don't think it's safe to assume the maximum human lifetime is a constant in these discussions.

While we wait for research to get there we can play with the other rocks in the vicinity. All those climate engineering ideas could be tried on Venus, for example.

If we could change that atmosphere (engineered bacteria and collision with an ice asteroid to lose some gas and gain some water?) and cool it down a little.. It's not that far out of reach.