I’ve actually thought for some time that the only way to colonize Mars would be to send people there on a one-way trip. Of course, you have to face the brutal reality that those who go are very likely to die in the attempt. Mars is not where we evolved and its environment is almost certain to have unknown hazards. But the back-and-forth business is simply not going to work.
In the end, I think Heinlein was right: space will be colonized by people we decide we’d rather dump off on another planet and let survive on their own. That’s how the Western World colonized the Americas and Australia. You combine some eccentric billionaires and an opportunity to get rid of political/racial/religious undesirables and we’ll spread through the Solar System in no time.