Marring the girls just because they told him to is also stupid. Especially the princess who forced herself on him, then gets jealous even though he said "no" originally.
Making establishment of a new kingdom boring must be a special gift of the author.
The reasons are 1) MC is too OP. There is never a feeling of danger or any tension. His companions are also never in danger 2) All problems are solved in a boring way. He only uses 2-3 magics, somehow he just becomes expert in martial arts even though he never trains. 3) Whatever the situation is, there is always some kind of deus ex machina ready.
MC is being stalked by a psychopathic summoned guy who is in love with him. That guy manipulates countries and tries to kill everyone who is close to MC out of jealousy.
The plot armor of MC is over 9000! Nothing truly bad ever happens to him or his companions. The plot is set up in such a way that all major events happen at convenient times so MC can find a clever solution or a pull one out of his ass because the time was just right for him to create a new monster
For example, I wanted to read more about the reunion with Ossan in the academy city, but as soon as they met, author switched to a different scene. Or the stupid 5-chapter cliffhanger about the "trauma of the past" that Norn was guilty about turned out just to be chuunibyou. Many other instances where events are stopped halfway through and later we return to the end after everything was over.
Like when the chick from another world masturbates at night and he thinks she is doing a secret training because he feels he heart rate go up.
Also the part with his sister is so stupid... You can guess what happens just from the already translated chapters.
1) When assassins attacked his home and the fairy killed them all, he thinks that they all left by themselves after she asked them politely because she said "they all left". Even though right before that she told him there are assassins and no one will miss them when they are gone and locked him in the room while she was killing them. For real? Mentality of 5 year old from 94 year old.
2) When he increases his mana to ridiculous amount, and people are telling him "wtf, your amount of mana is 10 times normal magician should have", he still thinks that he does not have that much mana. He also thinks that he is average in strength even though he is OP and that everyone around him should be the same. If he just looks at people around him, he can realize what kind of freak he is, and yet he does not.
3) His sword. Not only he trades a higher level artifact sword for the lower level artifact katana, he does not even freaking suspect anything. When his katana starts to perform above normal sword, he does not investigate it at all and just chills out still thinking it is a normal sword.