We are not reproducing nearly enough, our problem is aging and declining population, not overpopulation. We still have plenty of space on earth, even without seasteading and terraforming the deserts, and after that there are several empty planets waiting.
A) Overpopulation isn't only about space, it's also about resource usage and the wastes generated (including CO2).
B) you're free to live on the Moon or on Mars once(IF) it becomes possible but
1) the ticket price is VERY expensive
2) once the novelty has faded, there's a very real possibility that you're going to be very jealous of those who can breath 'fresh air' outside.
3) these colonies are only useful for humanity if there are self-sufficient but they need high technology to just continue to exist, so how can they become self-sufficient?
How are you going to make solar panels on Mars with only what is available on Mars?
1) If the population stays at ~7 billion ticket price will stay very expensive, but if there are several hundred billions, we'll build a launch system like "space fountain" or one of other proposed designs that are not economically viable yet due to low demand. This in combination with solar sails in space can make the ticket pretty cheap.
2) I don't think i would be jealous of those who can breath 'fresh air' , i already spend most of my time in a room, and with better VR games and large domes/caverns i don't think i'll miss the outside.
3) Again with large population on earth, there will be enough people who want to go to mars to create self sufficient economy there.
A) the amount of generated waste does not depend only on the population but also on the time the population lives at the given level of technology. When there are less people less resources are spent on science, and people end up generating the same amount of waste eventually, and do not develop the knowledge required to generate less waste. Another variable is the desire to generate less waste: as long as the amount of the waste is not threatening most of the people are not interested in generating less of it. Some see the problem sooner than others, but with lower population the number of these people is also lower and they have smaller effect.
If you look at the history of humankind you'll see that we were able to do great harm to environment even with much smaller numbers (killing megafauna, deforestation). the difference was that then we were not able to understand what was happening.
The crucial point is that the cost of developing new technologies does not depend, but reward grows, with the number of people alive. So when the number of people grows, everyone wins.
I believe the myth of overpopulation and the cultural norm to have less children, have caused great harm. Migration from poorer high birthrate countries into rich low birthrate countries leeches the most important resource the poor countries have, more educated and more active people who want to change the country. If developed countries were giving as many migrants as they take, we would be living in a much better world. Hopefully technologies like longevity therapies and artificial womb will be developed sooner than the population starts to decline.
(aside: it's funny how always similar things come up in batches https://twitter.com/elonmusk/status/1141977875567919104)