I recommend Man vs. Wild: I Have Danger Level Hints for you. This sci-fi-doomsday crisis novel tells the story of the protagonist Lin Xiaomu unlocking the [Peerless Card System]. When he encounters danger, the Spade Card will appear. The numbers 2 to A represent the level. 2 is the lowest level, and A is the highest, so he no longer has to panic. This book fits the plot of the novel you mentioned, and the description of the levels is very similar. In addition, the protagonist also left his hometown, but immediately returned after seeing that something had happened to his hometown. I hope you will like this novel.