Tensei Kenja no Yarinaoshi: Ore Dake Tsukaeru Kikaku-gai Mahou de Nidome no Jinsei wo Musou suru

A great sage has dedicated his entire life to magic, solely for the purpose of defeating the Demon King. But when they finally meet, the Demon King escapes to the future using reincarnation magic...! The great sage follows suit and is reborn 500 years later as Ren Strain, only to find that magic in the future has declined to an astonishing degree—!?

(Source: Starts Publishing, translated)