Apparently a country just happened to have 80k troops within a few days of march of another country just idling around. Either the country was tiny, in which case the number of troops is ridiculously high, or they had some transportation that wasn't mentioned, or even likely considering how low level the world is.
A part of my country wants to establish their own? Send the army that was in the previous spoiler. The army was destroyed? It must be a fluke, gather a bigger army of slaves and mercenaries. Oh gee, you couldn't, say, send someone to find out what happened? Not a semblance of intelligence makes you doubt the quality of the future villains. And that's without going into the inconsistence of building the new army had.
"The attack that reshaped the mountain? I can fire 100 at the same time." "Mithril is rare? You'd need orihalcum, mithril is such a boring metal I plated the outhouse with it." "The legendary dragon? My weaker guildies went for practice and now it's sleeping in a dog house in front of my castle." and so on.
The MC goes to cheat with another woman right after getting married to his harem to make sure his five wives didn't just coincidentally get magic boost after receiving his... fluids.
He can create firearms of any kind essentially at will over long term. Short term is limited by his mana, which is still pretty allowable. They are as powerful as you'd expect against fantasy people.
He can also create legendary monsters with certain relatively lax conditions at will. Other dungeon masters have 33% chance of getting A rank with the same method (B otherwise), MC pulls S rank at 100%. Yeah.
Now combined these two points make him broken OP. At one point 3 other dungeon masters ganged up on MC under favourable rules to them and had powerful monsters supplied by older dungeon masters. They got defeated with barely any difficulty.
When they get to city building phase it's used to pull workforce out of nowhere. Need more smiths? Here's some master smiths out of thin air. No effort required.
There's the fox girl, dwarf smith that is good at building stuff and flying elf. They all love their "father" and want to be of use to him. Oh, and the male undead butler dragon and the yet unnamed water girl. They also love their "father" and want to be of use to him.
MC was getting corrupted by him being a demon lord if he used a power. How was this solved? Have a subordinate call a powerful subordinate to whack the MC every time he seemed to go out of control until he realized that going out of control was silly. Normal subordinates cannot harm demon lords, but conveniently he had just acquired a subordinate powerful enough to have a subordinate of their own of that level.
MC is giving off great deals one after another and no one is suspicious of things being too good to be true. Say, the casino. MC decided that his city needed a casino, so he went for a business partner. 90% payback rate wasn't good enough, so MC built the facility and compensated with the city's income for the partner to handle the casino. Yeah, definitely not a suspiciously good deal. Or the time MC just claimed to be a god to counter another hostile religion encroaching his city and everyone in his city simply accepted it.
Our MC had a private tutor coaching him in school subjects. He managed to reach university level before entering school and his father knew it, therefore he had to go to school because reasons.
MC is way too OP for the story to go anywhere. He can do just about anything with his magic, including a spell like "That sort of thing should not be". Yes, you read that right. If that wasn't enough he's also super strong and has the strongest summoned beast. To negate any potential conflict he's also super hottie and comes from prestigious and wealthy family. Armed with these powers our MC leaves on a quest to become the best helper for his father's paperwork. No, seriously.
Pointless PoV chapters. A fight is played from the MC's PoV. Then the kidnapped friend's. Then the bad guy's. Then the random dog who happened to stand nearby. Well, not the last one but it might have been better if it did. I'm still confused why that firework maker needed to have two chapters.
The summoned beast played barely any role in the story. It got summoned, turned into human and appeared in about one scene in a minor role that could have been done without.
Have you seen more than one slice of life school anime series? You now know half the plot. This is also why it's not that great as a slice of life.