I Joined the Human Adjustment Bureau with a System

This book features science fiction, an apocalypse setting, a System, multiple elements, and no reborn or transmigrated protagonist.

System abilities: freely control the body's hormone secretion and metabolism.

System upgrades: the System can be upgraded continuously. Beyond its basic abilities, it can eventually control a person's senses at will, and even their brain waves.

The highlights: make money, while helping a superstar stay young forever; extend the lives of international bigwigs; keep tycoons in top shape; and, thanks to the rarity of the System's abilities, become a formidable combat force...

Wait, you didn't think any of this was free, did you?