Watashi no Kokoro wa Ojisan de Aru

Without friends, lovers, or family, Haruka continued to live a life of inertia, but one day, he wakes up in another world as a dark elf beauty! Haruka can't contain his excitement at his new environs. However, there's no way an old man who has given up on many things and has been afraid of interacting with younger people and women can immediately become invincible... Despite having inexhaustible mana and a sturdy body, he overthinks things and is indecisive, but by meeting cherished friends and developing with them, his "heart" also grows in this heartwarming fantasy.

(Source: Shufu to Seikatsu-sha, translated)