I recommend 'The Races of the Galaxy'. It portrays a world with billions of races, technological civilizations, Immortal Cultivation civilizations, martial arts civilizations, and so on. Also, the title of the book has the word' galaxy 'in it. I believe that it has a similar atmosphere and theme to' vastness'. In addition, the protagonist in the novel also grew up to become the master of all races in the starry sky, which also matched the feeling of "I am the vastness". I hope you like this fairy's recommendation. Muah ~😗