This is better than it has any right to be. I mean, sure - it follows a lot of the isekai tropes, but it also does something very different in that the MC actually confronts and is cognizant of the consequences of his interference. It started off as a better-than-average fantasy fuckfest, but it rapidly grew out of its niche and has started truly addressing the world its in.
Kousuke as an MC is also a breath of fresh air in the genre. He's not one-dimensional like so many other isekai protagonists. His cheat skill is cheaty as fuck, don't get me wrong, but it's quite divergent from the normal "OVERWHELMING POWER/STRENGTH/MAGIC/KNOWLEDGE" bullshit that gets dropped so often. He has an inventory that operates on game logic, and while he abuses it for practicality, it is played for comedy just as often.
Furthermore, while this is very clearly edging towards harem, it's not a mad dash to the fuckish line like most would be. I can't exactly say that involvement with other women has been 'slow', but the pacing is a lot less frantic and honestly comes across as situationally appropriate in a world where sexual mores allow for multiple wives.
Sylphy is also really cute as a character - she's strong and intelligent, and is very clearly a capable leader, and it's pretty amusing to see her wrestle with a sexual relationship and anything that might be considered overly feminine. Seeing her get embarrassed post-coitus when she was pretty into it was hilarious, and I'm glad that it happened only a few times - rerunning the same gag would have cheapened it.
Aira as a second partner offers a very different kind of character. She's very much an intellectual, and I thought her struggles with coming to terms with Kousuke's ability to just completely ignore natural laws were both funny and highly accurate. Her characterization is significantly different from Sylphy, but there's enough crossover to really allow for her to integrate in the relationship instead of just being an 'also-ran' girl. It's pretty impressive to see effective relationship dynamics play out here, and while Sylphy is clearly 'best girl', it's much more balanced that I would have expected.
I was not particularly thrilled at the Harpy sleep gang-bang bit, though - strikes me a bit too much like rape, for all that it got glossed over - and I get the feeling that the author realized it was a misstep for how quickly that faded away until he had a chance to correct it story-wise with an effective 'do-over'.
The art is very good. Very good, indeed - not all the designs of the beast men are great, but considering that Qubi/Kyuubi is ultimately revealed to be a bad guy, that may or may not have been deliberate. I kinda feel like Qubi's antagonist state might have been reactionary based on the author not being pleased with the character's design, but I don't know that for certain.
All in all, I'd put this as an 8.5, possibly 9/10, with a lot of that coming from storyline, balance, and actually introspection.