I have a better idea:

1) Populate Australia and Siberia first.

2) Convert some oceans into land (or cover oceans with land).

3) Colonize Antarctica.

If you still have not enough land to play with -- consider spending money on playing with Moon, Mars, and Venus.

If you want to populate somewhere, how about Canada and the unpopulated, beautiful plains out there to the north. It is already colonized but has virtually zero population density. The fact is people don't live there for socioeconomic, environmental, and political reasons. I don't think populating Antarctica and Siberia holds much value except as an experiment. There are plenty of places for people to live in.

If you refer to turning Siberia, Northern Canada, Antarctica into a useful piece of agricultural land, yes that theoretically has value, but I doubt is practical given the risks to our climate. Plus I hope advances in agriculture and mining will not require terraforming those regions.

I think the main value of going to the Moon, Mars, or Venus is to have people be on multiple terrestrial bodies to hedge some extinction risk, to begin some sort of space exploration infrastructure in the future, and maybe find significant economic value in mining. We need those benefits even if we haven't colonized or terraformed all of Earth.

The main value of going to the Moon and Mars is shallower gravity wells. The Earth is the largest terrestrial body in the Solar System - the most expensive place to live in terms of getting somewhere else, except for the gas giants where we can't live anyway.

Of course, in terms of life support it's by far the cheapest place around.

You are laboring under the notion that people want to travel to and live on other planets because there is not enough habitable land on Earth.

That's not the case. People want to go, that's all that matters. Eventually technologically and economically it'll be feasible for that want to be met, and it will be.

This is kind of "putting all your eggs in one basket", though. If we're going to be terraforming and altering global or continental environs, we might as well do so on the Moon or Mars so that we have some insurance in case of nuclear war/meteor/other apocalyptic death scenarios.

If you go through all the trouble to adapt to living in space for travel between the planets, why bother landing? Colonize the asteroids and short period comets, which are full of resources and much easier to access. No landing vehicles and ascent rockets are required. The gravity is so low that a spacecraft optimized for interplanetary travel can dock with them, or loiter nearby and transfer people & goods with space tugs.

Of course, it's a matter of preference. Many would still prefer to live on a planet. A dispersed civilization that includes the small bodies would be even more resilient to any one collision. More eyes out among the asteroids would also increase the chances of spotting potential collisions.

1) It's much cheaper protect yourself from nuclear war and meteors while staying on the Earth (e.g. digging down) than move to the Moon. Moving to Mars/Venus is even worse than Moon.

2) If you want to "insure" our civilization from disappearance -- focus on developing smart machines that can travel anywhere and carry our civilization accomplishments to other places. Sending people to other planets/asteroids is extremely inefficient in comparison with other options.

As charlief said, it's all about existential risk.

Not cool enough to get the public to agree to put lots of funds into.