Fun Territory Defense by the Optimistic Lord
An unnamed 30-year-old salaryman learns the hard way that the more and better you work, the more responsibilities the company dumps on you, until one day, barely making the final train home, grabs a cup of hot ramen and goes to bed only to wake up reincarnated into the body of two-year old Van Nei Fertio, the fourth child, and fourth son, of a Marquis house in a world of swords and magic. He spends the next 6 years hailed as a prodigy due to mastering the way of the scholar and the sword, but on his eighth birthday, rather than get one of the Four Element magic talents Van's father was expecting, the kid ends up with the generally accepted to be useless "Production Magic" talent, and the perception is actually justified, since production magicians can only create stuff they understand, still need materials, and the stuff they create are usually inferior to stuff built by non-magical means, with having a poor mana pool to boot. Van's father considers executing him on the spot until his eldest brother instead has him Reassigned to Antarctica by having Van take over a distant, backwater village on the edge of Fertio territory that's been badly neglected due to many factors beyond the Fertio family's control. Well loved by the townspeople and family staff, Van is followed by his maid, his slave, the family's top knight, and the head butler who just happens to be a retired general and warmage. It's not long before he flips the local common sense on its head by showcasing just how mighty production magic can be with a guy who does his homework...