Dreams.

This sounds like a blueprint for old-school science fiction.

Work out the details, make a coherent plot and you could even write a book.

Roughly the middle of the XX century. Scientists find out that the Solar System will soon be F'd. Why? There are many reasons. From exotic ones, like a rogue star or an accelerated transformation of the Sun into a supernova, to a neutron star and a stream of radiation heading our way. We need to get out of the system, and fast. Scientists and governments of all countries unite, ignore the Cold War, and work on preserving civilization. They decide to make a run for it, but building a huge generation ship is long and difficult, so they decide to use an existing celestial body as a generation ship—the Earth itself, the Moon, Mars, or a moon of Jupiter/Saturn. Humanity's migration begins, calculations are made. Let's say a new star enters the system, and due to gravitational changes, the Moon is thrown out of the system into the cold; then we take it, terraform it, and relocate. We complete the process, set up resources, atmosphere, water, gravity—essentially a mini-Earth—and beautifully blast off into space.
But, as with all generation ships, everything goes wrong. The main core—scientists, guys responsible for the planet's movement—stay in underground bunkers, and there are quite a few of them, plus workers and all that. And ordinary people live on the surface, under a dome. And they slowly start to go feral. Not completely wasted, but forgetting technologies, considering them magic, cities as ruins of the ancients, living in wooden houses and all that. The tone of the narrative becomes fantasy—the same plague with the search for a magical cure—but at some point, there is a clash between surface dwellers and underground Wizards, and the latter realize they got too caught up in their own problems and ignored what was happening above. They start paying attention, progressing civilization back again.
Slowly we fly toward the goal, closer to the center of the galaxy. The planet is shaken—more stars, gravity, and interesting radiation; we have to maneuver carefully. It turns out that the star they were flying toward also managed to get F'd. They run calculations and it turns out it would be convenient to enter the orbit of a black hole; they attract a lot of interesting things, all sorts of energy and matter, and one can sit there for a long time stably and catch the goodies flying by. Everyone is shocked at first, but in the end, that's what they do and they become a fashionable successful civilization.
I see a cycle of novels.
1. Fantasy, POV of surface dwellers. Modern devices and technologies are described as magical artifacts of the ancients, clumsily and mysteriously, but the reader guesses what's what; vigilance is lulled by various monsters, sorcerers, and ambiance, and at the end, there is a meeting with scientists and then—plot twist.
2. The era of progressorship. People have already settled in normally, but some are xenophobic toward those from underground; they try to negotiate peace with them, but "pyash-pyash" happens. A connecting link so there isn't a sharp transition between books.
3. The part where they are at the center of the galaxy and enter the orbit of a black hole.
4. The future. Xenos find the remains of Earth, where life once was but has since vanished. Suddenly, a ship from an ultra-ancient, ultra-developed civilization that took over the center of the galaxy arrives—a civilization whose greatness is the subject of legends. And it turns out that Earth is their homeworld. Perhaps this is exactly where the flashback with the backstory described at the beginning happens.One or two more volumes could be added—how they managed to get back into space, towing meteorites for resources, completing terraforming.
Something in the spirit of Asimov's "Foundation" should result from this eventually, but less large-scale and epic.