I Don't Want to Protect the World, so I Decided to Train a Successor

(Protagonist retires) + (Protagonist is invincible) + (Multiple characters) + (Time travel) + (Grows an apprentice) + (Monster invasion) He came to this world due to a car accident, and was given the qualifications to practice and the obligation to protect mankind by the system. As a result, when he saw the primitive people who had just learned to drill wood to make fire, Lei Fan's expression was as uncomfortable as Cao Dan! But there is no way, in this world where monsters, monsters, and even aliens can come at any time, if you don't get stronger, you will die! In order to survive, you can only complete the mission and become stronger at the same time! So he diligently protected mankind for five thousand years, until... "What? My strength has reached its peak and you are leaving me?" "Holy crap, I've been living with this system for five thousand years, but I'm not familiar with it at all!" "But now that humans have developed to a certain level, should I consider training a few successors so that I can retire?"