It seems like you're asking us to come up with a 30 year 2.1megaton mass budged moon/mars/venus colonization plan. There's likely to be more than one valid plan, and any plan is likely to be far more complex than can be reasonably answered in a single post. Can you edit your post to ask a much more specific question? Perhaps clarify the objective, focus on a single colony, and describe the budget available for for non-launch expenses. Simply asking us to come up with any plan we personally think is logical is far too subjective a question for this site.

Questions with many valid answers aren't permitted on this site. Your edit makes the fact that this question has many valid answers more explicit. We're not a brainstorming site. Asking us to come up with a collection of plans is not a suitable question. Instead of leaving it to us to make up plans for you, why don't you try coming up with a plan on your own. If you have a specific problem creating your plan, ask us for help resolving that particular problem.

Please pick a threat and pick a plan and then ask specific questions about that plan (after checking that they haven't already been asked on this site). For example, at the moment you are offering an AI rebellion as one of the possible threats, but an offworld colony is probably going to be more dependent on high-capability computers and robots than people on Earth are, making it more vulnerable to that threat.

Ok, I think the AI threat is more relevant, I'll go with that.

@sphennings So can you help me out a bit with the nuances on "Questions with many valid answers aren't permitted on this site."? Other questions seem to have many valid answers such as: worldbuilding.stackexchange.com/questions/12348/… and worldbuilding.stackexchange.com/questions/253369/…

Those questions should both be closed under current site policy, thanks for bringing them to our attention.

@sphennings I reviewed the rules here: worldbuilding.stackexchange.com/help/dont-ask and edited the question. Could you reread the question now and let me know if it now meets the site's criteria?

Your post still ends with you soliciting plans. If your core question is "where is the best place for a backup colony?" the entirety of your post should be in support of that question. It should be obvious what you're asking. Leaving the remnants of old questions kills clarity, and makes it harder to argue that the questions issues have been resolved. Do an editing pass to remove any extraneous material for clarity.

@sphennings Did another pass. Which paragraphs (1-7) do you feel are extraneous and not really helping the question?

The question still reads as plan oriented not location oriented based on the information you provide and how you talk about it. Instead of giving us a budget you’re asking us to figure out the budget. Ask yourself is this necessary to determine the location and delete everything else. Remember this is your question.

With that kinda cash maybe just fix Earth?

AI raises a ton of very real ethical questions. When "industry leaders propose that AI poses a risk of extinction", what they're doing is diverting public concern away from real ethical urgent considerations by flooding the debate with science-fictional scenarios in which the villain is AI (and not the people who use it).

The biggest threat AI poses to human life is not from launching nukes, but from poisoning our information environment until digital civilization breaks down. Any habitats linked in cyberspace - regardless of whether they are on another planet or a space station - will be just as vulnerable as if they were on Earth.

@sphennings I cannot find text in the dont-ask page forbidding questions with many valid answers. In fact, the opposite is true: Questions with only one valid answer typically don't need to be asked, or they only expose a lamentable lack of information on the OP's side; any way, they are boring. This is an interesting question. And even if such text existed: Why be so strict? This is a place to throw around ideas and have fun, not a clean room. We can be a bit messy, it helps creativity.

Frame challenge (hello Elon): Yes, it is a fantastic idea to use 30% of the global military budget to save lives instead of destroying life. But by far the most efficient way is to spend it on Earth. We must preserve Earth at all cost, and there is no place in the known universe so custom-tailored to our species. A penny spent here is more effective than 1000 dollars spent on Mars.

@Peter-ReinstateMonica It's a backup strategy. Ever heard the expression: Two is one and One is None? Applies to planets as well.

Recent book: A City on Mars. Summary: Probably can't work in the next 30 years.