The story takes place in a sword and magic world. A young nobleman is trying to restore/rule his territory. So there is some economy and agriculture and trade stuff. I remember him using wood/bamboo slates like the old fashion Chinese for "paperwork"/accounting because it's cheaper and easier than using/making paper. There is constant money trouble and on of the beginning chapters is him and the men of his territory hunting monsters to make money. They show the after math in which they set up a healers tent and a peasant lost use of his hand but made enough money to feed his family that winter. A labyrinth/dungeon appears under the castle, which is troubling and also potential profit. Dungeons grow without rhyme or reason but the monster parts and the trade make any territory with a dungeon wealthy and some of the noble's neighbors may try to take over his land if it was found out. So he solos the dungeon in secret. The reincarnation part comes in prior to the first chapter the young noble had been casting a spell on himself repeatedly with multiple failures it's a well known spell but a legendary spell that no one in living memory has successfully cast. Those who do have their names go down in history. The spell downloads the memories of a parallel universe version of yourself into your brain. Say one with modern knowledge from our science world that would be impossibly advanced to their feudal magic world. Or maybe a version of yourself from a martial arts world who knowledge would make you a military genius or the like. He got a version of himself from our world. Note, the alternate Earth world him is still alive and unaware of what happened just his life knowledge has been copied and overwritten the personality of the young noble. Even he himself wonders what he was thinking to kill his old ego with a spell, desperate and grasping at straws, I guess. I also remember their was an OP girl who shows up in his secret under the castle dungeon.
Any one know which story this is? I think I remember it being full color so maybe a Korean webtoon? Maybe I'm not remembering it right?

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It's webtoon Maze Age Z