Very generic power fantasy with the sole purpose of teenage japanese bois to self-insert in the story and pretend they are an OP Edgy MC with a harem, I mean the isekai aspect adds nothing and it was obviously only shoe-horned in for self-insertion purposes... even since the start the generic looking MC already had his very diverse looking group of girls in his harem, is not wasting a second to pander it's intended pubescent demographic.
Nothing wrong if people like stories like this one, I also have my guilty pleasure selection of trash manga, just don't lie to yourselves and pretend stories like this one are not that bad.