Fuguu Ouji wa Tensai Renkinjutsushi: Koutei nante Gara ja Nai node Teimai wo Kawaigaritai @comic

One day, a man longing for a warm and loving family gets reincarnated into the magical Iscalion Empire... as Prince Asha, the unwanted eldest son. But he's too "tainted" by his bloodline to be an heir and has been separated from his younger brothers since their birth. So, he gives up on attaining the throne, embraces freedom, and dives headfirst into alchemy as his personal playground. But with his overflowing talent (and memories of modern science), he ends up inventing world-shaking tech by accident?! Even nobles get smacked down by his "inventions" Before long, he's unintentionally dominating the imperial succession war.

"All I ever wanted... was to be called 'Big Brother' someday."

A chill, genius big-bro conquers the empire—for the sake of his younger brothers.