I Was Found to Be Competent By a Heroic Female Knight and Lead a Beautiful Harem of Knights.

Gaikaku, a genius mage with unparalleled magical prowess, lives in quiet seclusion under the employ of a gluttonous regional lord, Borick. While earning pocket change through secretive assignments, he devotes himself freely to magical research, surrounded by a diverse group of girls from various races. However, after taking down a certain elite elf, his incredible talents as a mage begin to draw attention across the land...!? The ultimate mix of tactical mastery, magical engineering, and knightly valor—a harem fantasy manga adaptation begins!

(Source: Kadokawa, translated)