There is an old proverb that advises: "The person saying that it cannot be done should not interrupt the person doing it."
I am an artist so my credibility is open to question on this matter. My one claim to any kind of expertise is that, once upon a time, I illustrated a book called The Millennial Project, by Marshal Savage. It was fun, and I would like to think that I learned a lot from it. If I am allowed a brag or two, the highlight of my efforts was receiving this signed photo from Arthur C. Clarke with my illustration of a terraformed Mars hanging behind him:
https://www.deviantart.com/keithspangle/art/Arthur-C-Clark-282923306 .
Since then, I have, on occasion, returned to the possibilities of the future as a subject for my paintings, and to add depth to the artwork have given some thought to our future both here on Earth and as spacefarers. My advice: Do both. Restore the Earth and become a Space-faring Civilization.
We have, in popular fiction, two resonant models for both goals: Lothlorien of JRR Tolkien's Lord of the Rings, and the Federation of Gene Roddenberry's Star Trek. I will leave the details to people who actually have the technical expertise to start making them a reality; my concern is more with the images of the future that I would like to see. While my work will construct no L-5 colonies or plant no trees, perhaps they can inspire others to do so...and these days we are sorely in need of positive inspiration. (Here is another link, this one to the series of paintings that I have done using the future as a subject:
https://www.artstation.com/keithspangle . Never mind the "mature content" caveat...it is nothing terrible)Perhaps some of my work will trigger an idea or two.
Oh yes...and one other thing: Get over the idea of having to confine ourselves to planets. Build L-5's, use mined out asteroids. Build ships the size of moons.