We disagree by several orders of magnitude
"I do not believe we have an infinite period of time to colonize space based on limited resources here. My best guess is 50 to 100 years and fear it could be as little as 20. "
My best guess is tens of thousands of years before we start running a significant risk of planet-wide human extinction from asteroids or comets. I see no evidence that we'll run out of resources that will keep us from going into space.
Even if you think the time frame is as short as 100-200 years, the logical response is to invest in basic technology that could get self-sufficient colonies in space in several generations, not the useless or super expensive flag and footprint missions.