> Humans need to leave Earth long before the Sun turns into a red giant.

We have billions of years for that. Everything else we can deal with much easier here than we can on mars.

> Without volcanoes to recycle carbon into the Earth's atmosphere, carbon dioxide levels fall to the point where photosynthesis is no longer possible.

And after that the earth will still be more hospitable on mars.

> Good luck in the bunker.

We'd be living underground on mars, any sort of terraforming possibilities are in the far distant future, possibly never.