Do we actually know how to build a self-sustaining colony? Last I heard, we still had fundamental science and engineering questions to be solved even if we suddenly had an unlimited budget.

Things that come to mind include building a sustainable closed ecosystem, figuring out a long-term power source (is there uranium on Mars? Nuclear reactors run for a long time, but we can’t rely on fresh fuel rods being shipped from Earth), and planning for enough industrial base that things like mining the necessary uranium, digging tunnels, housing construction, etc aren’t colony-ending problems.