If you guys want to remove any confusion you have about the MC.
Mind you, I stopped reading the novel translation a long time ago. But no, the MC really is just stupid. He's super paranoid but he does have a power beyond anyone else and that is the ability to make use of artifacts. Everyone else drains their powers just trying to power one, but he carries multiple of them, to the point where he continuously drains the entire coffers of the guild just to buy expensive artifacts and each of those artifacts are incredibly powerful. That's part of why it was a big deal that he could use the sword artifact someone else owned like it was nothing.
For example, if memory serves me correctly, he owns and has equipped multiple artifacts of absolute defense, that will negate any attack that comes his way no matter what that attack is, but it takes him a while to recharge them.
But he himself, as a person, has no skills, no endurance. He's just someone whose power is to make use of god level gear, so he's entirely gear dependent. In everything else he is pretty average. If there was some sort of reveal later down the line in the novel that changed that, I haven't gotten there.
Think of him like Iron Man, except he isn't a genius, he's just a hoarder who's dense and because he grew up with childhood friends who were exceedingly powerful and love him so much that they all want to spoil him and meet his expectation, his sense of "normal" is screwed. He expects the average guildmate to be able to achieve S rank type tasks with ease, because that's the world he knows.
So mix the fact that he is physically and mentally average, has a skewed perception of what is normal, is surrounded by super loyal super powerful childhood friends that make up the core of his party, was forced to take the position of leader so his word is law forcing his skewed perception of reality on everyone else, and the draining of the entire fortune of the guild to hoard all sorts of super powerful artifacts that fits well with his singular power of recharging and using nigh unlimited artifacts and voila.
That's your MC, and that's how the guild became the strongest guild. And funny enough, because of his dense nature and his genius friends, he also happens to be the force behind illegal underground activities without knowing it. So he's both the boss of the strongest guild and the boss of a notorious underground group, the latter of which he has no clue about but those involved fully believe he's 110% in and plotting some big brain grand scheme a la Overlord, sasuga Ainz.
The manga helps alleviate some of the issues of the MC being super dense. Mainly because you're not in his head as much, so the story goes down easier.