The novel it´s almost a masterpiece, the "world" and the overall plot are amazing, the thing that ruins it, is the MC and all the plot devices around him, that make everyone around him suck so he can shine, first, everything is too convenient to the main character, he was the only one reincarnated with two cheats (ok, because he saved a kid, only him had ways to help someone on the crash?), then to make worse, BOTH his cheats are strong and overpowered, most people got useless or situational ones, he got two overpowered useful cheats (analisis of stuff, see other people character sheet and enhanced xp gain), then after all that unfair advantages, he is born a noble (since he can´t even have to struggle being a s*ave or with poverty), has a nice family, and all, everything convenient, then to make things worse, the guy has photographic memory, he remember everything he learned of chemistry and science from school and college, and techniques from his time at SDF, he for some reason know all components of weapons (and have them memorized), he is good at everything, only the main character out of 49 people reincarnated learned the "exploits" of the system (the mana thing, and was a easy thing to figure out, more than half of the reincarnated ones should have it), he too was the only one able to apply modern knowledge, out of 49 people everyone was s*upid and in the end, everyone he recruits has skills that benefit him to become more OP (like the map girl and the guy that help him manufacture guns with no need of precision tools). The antagonists are obviously underpowered compared to him, even the strongest ones would be curbstomped by him easy, so there´s no tension, there´s really no challenge for the MC, he will always win, easy, being better than everyone at everything.

The story follows a 45-year-old sales manager who dies in a train accident and reincarnates along with fellow passengers spread out throughout a classical isekai world of Orth, with past life memories and a couple of extra cheat skill (each reincarnated person got a random one, but the MC saved a kid during the crash, so he got a second one as a bonus). We watch him grow up as a second son of a landed knight (1st volume) and then follow him on his quest to delve into the nearby dungeon (2nd volume) and gather enough riches to create his own country (3rd volume), while at the same time gathering allies mostly consisting of other reincarnated people he met during his travels.

gather under a guy who reincarnates as the heir to the neighbouring kingdom (possible future allies or foes).

If you were to go into the past right now, could you immediately utilize modern agriculture techniques to improve how farming is done? Could you make a pair of shoes that doesn't fall apart or cause injury to your feet? Could you make a gun?

Coming from a modern world, the MC has many modern concepts in his head, I understand. But a few years in the military + university level education does not result in several lifetimes of specialized knowledge and skill sets that include engineering, chemistry, machining, cobbling, and a dozen other professions being put into your brain.

Also, rubber body armor 1 CM thick would be extremely heavy. Please consider the weight vs actual protection value. Mail armor is ancient era tech, and should be readily available at 'relatively' low cost in the era the story takes place. Pound for pound, I can't see any way in which rubber would perform better than steel of equal weight, be it vulcanized or not.

If you remember she was an assassin/s*ave sent by a kingdom (forgot already). When she pretended to be from xx village she was adopted by some noble in MC's kingdom. She then proceeded to become a maid for MC's family. She is grateful that she lived a peaceful life without danger and war. THAT is what she's grateful TO the family of MC. However, she was later FOUND OUT that she was a spy by MC. MC decided that he wouldnt kill her, and instead continue the relationship while protecting her. I have not read what he did later on but I can only assume that he finished his plan of faking her death. This in turn allows Myun to continue to enjoy her peaceful life and not go back to being an assassin. THAT is what she is thankful for.