Isekaigaeri no Daikenja-sama wa Soredemo Kossori Kurashiteiru Tsumori desu

At the age of 15, Saitou is reincarnated into another world, becomes a great sage, and defeats the Demon King. Immediately afterwards, a door connecting the other world and the real world opens, and Saitou returns to Japan with the power of a great sage. However, 19 years has passed in Japan. Chronologically preserved, Saitou meets his childhood friend, Kanako, who is now a single mother. It seems that there is an enemy who is aiming for Kanako's life... The slow life wannabe story of a great sage who came back from another world begins!

(Source: Kodansha, translated)