Summary:

Haruto is an ordinary Japanese student who is tired of constant work and study. After getting trucked, he is reincarnated as Phil Graceheart, the youngest child in a royal family. When Phil is three years old, he gets a head injury that causes him to remember his former life. Armed with the knowledge of his past self, he is now on a mission to live a completely relaxing and carefree life.

Notices:

Raw: https://www.alphapolis.co.jp/manga/official/564000194
https://mangadex.org/title/25641/tensei-ouji-wa-daraketai