That ritual is kinda the problem. He is prurient to the exclusion of the world around him.
I agree this is a problem for story telling, but its also becouse of the manga skipping out on the allready limited world building.
The story was to begin with pretty heavy on smut and removing the parts that isent did not help much.
It might change in the future chapters if we are lucky.
The previous arc along with the one just after he gets the 3rd girl is REALLY heavy on it, more so then most others.
(in said arc he unlocks a secret human only class by doing an ungodly amount of sex on a regular basis just to drive home the point what a mad lad he really is.... )
But its likely we just got to deal with the excessive smut.
He only goes out to buy or sell and he treats people like NPCs. Oh, 30% again ho-hum. Which is fine to an extent, that's who this MC is.
This is however not true at all.
He is actually treating other people fairly well and knows for sure that the people around him are real and not just NPC´s, the bandit arcs whole point was to establish that fact.
Again, the novel does it better but he were haveing some pretty heavy emotional issues both before and after killing people.
He might have become a bit more jaded to killing, but he isn't heartless by any means.
As for the selling discount, its not expressed as much here in the manga, but he notes it for himself as he wants to figure out why and how it works.
The 30% discount is a ability to mess with other peoples ability to calculate, pretty much everyone in the world uses a "math" skill to count bigger numbers.
So when people use the skill to count his ability interferes with the math skill and changes the number.
He is going to meet people later on who dont use the skill and so he dont actually gets the discount, but it's a minority.
This is also why he needs to buy at least two things so people actually use the skill, as if they only sold a single item it's pretty obvious what the price is.
It's frustrating, though since I've been waiting since the chapters that got deleted for the MC to realize that the fact that he knows how many sockets are in a thing means he knows how many enchantments a thing can take.
Yeah thats all the main focus on the next arc, IIRC up until this point he didn't actually know enchanting equipment was a thing.
This is why he picks a dwarf smith for his next party member, she have been on the cover for quite awhile xD
As for your final points about the dungeon being bland, i 100 % agree.
The dungeon diving don't really become very interesting until he gets the 3rd girl into the party, and even then its still pretty bland in comparison with the world above.
The dungeons is a main focus off the story but its one of the worst written parts too.
But they become less and less important later on in the story.
He is forced into dealing with stuff above ground more and more because of his personal connections and growing reputation / needs of his party.