We also have a MINOR problem unless we find a new way to propel rockets.

Did you know that the Saturn V rocket burned 15 tons of fuel PER SECOND for the first couple of minutes to get 3 astronauts to the moon? Part of the problem is the rocket needs to carry that much fuel so it need to have huge propulsion capability just to carry the fuel load. Things are a bit more efficient now but we need to be thousands of times more efficient.

I think if you do the math and figure out how much fossil based fuel we could manufacture only a few thousand humans could leave planet Earth.

Nice to dream but to make the dream realistic you must connect all of the dots. We might preserve human DNA by having a few hundred or thousand people migrate to a new planet but the rest of humankind will be left behind with the mess that has been created.