Enough with This Slow Life! I Was Reincarnated as a High Elf and Now I'm Bored: Volume 6 by rarutori

In the west-central region of the continent, Acer finds himself drawn to the Kingdom of Inelda. While it was once ruled by humans, rumor has it that the region’s elves gathered there and drove the humans out to establish their own nation, seeking protection their forests could not offer against the spread of the Western religion. The elves there ask our high elf to help them build a thriving kingdom for elves that can defend itself against their enemies. Acer promises to stay for ten years before he returns to his search for Win in the Far West.