"The threshold for a self-sustaining city on Mars or a civilization would be a million people. "

That number seems highly speculative ... But afaik, there is no self-sustaining city on earth, so I doubt there are more solid numbers on that topic. Economic cycles are quite complex, thats for fure.

But aside from that, I think the much more realistic plan, than building and leaving in a huge fleet at once and then see how it goes, would be sending robots first and let them build at least a rudimentary base first and mine resources and teleoperate the whole thing from a save distance ... if that works, we might be ready for colonisation.

And the needed technology is evolving right now on earth.

Maybe less spectacular, but more realistic in my opinion.