A good starting point is Gerard O’Neill’s 1977 book The High Frontier.
> It currently costs approximately $4,000 per kg to transport anything into space, that's current bargain basement SpaceX pricing. It also consumes non-renewable fossil fuels to do so.
There are plenty of materials in the solar system that aren’t at the bottom of a gravity well. It’s just a question of developing ISRU infrastructure. The fossil fuel problem is already solved; all of the newest rockets use methane fuel, which can be produced without fossil fuels.
> Let me know how you plan to figure out planetary terraforming, gravity manipulation (so we can live on planets with vastly different gravity), planetary climate management, off-earth mining. How many of these problems have been solved even with 1-5% progress?
Half of those aren’t serious problems. You don’t need to terraform anything to fit quadrillions of people into the solar system because you can just use small bodies like the moon as material for space colonies. You stated yourself how expensive it is to get out of the gravity well; why go back down into another one? The space colonies—which can be built with known materials, at scales approximating that of a US county—would rotate, providing artificial gravity through centrifugal force. ISRU (“off-earth mining”) is a very active field of research.
The obvious next step is to industrialize the Moon and build a mass driver out of lunar materials that we can use to ship lunar materials into cislunar space for construction. The best discussion of how to do the practical next steps that I’ve encountered is Ian Long’s YouTube channel, Anthrofuturism. (https://youtube.com/@anthrofuturism). The best starting point there is “How To Develop The Moon” (https://youtu.be/WZN2xXMb28g), largely based on his book, also titled How To Develop The Moon (https://a.co/d/0dn84hJa).
There’s a beautiful future available to us if we can escape the bucket crabs.