I'm a big space fan, don't get me wrong. But your exuberance uh, needs tempering.
More or less those things you mentioned have solutions and they are getting better.
https://www.health.harvard.edu/blog/measles-is-making-a-come...
Right now, the greatest threat to our survival and prosperity is humanity itself.
What if expanding helps us coexist peacefully? Maybe being in a crab bucket with no frontier is part of the issue.