In the world of entertainment fiction, it is not correct to classify things as objectively good or objectively bad, what matters is provoking strong emotions in readers.
Those bad isekai that you are citing are famous because the readers of those works feel pleasant emotions reading them...
The best fight scenes that I have always read are from novels by Robert E. Howard, he had a talent for making them frenetic and adrenalin-inducing, I recommend that you read his stories of Conan the Barbarian.
I hate fanfics.
I can respect that you use stereotypical and cliched characters and stereotypical and cliched worlds, but please, do not use characters and settings from another author.
It's not difficult, I could already do that with series, movies and animes, although if the author hasn't published for a long time, I would have to read his or her story again to catch up.
I don't think there is a problem, I'm sure I've already seen one here that has a theme of Nazi protagonists and another where the protagonist wants to spread Soviet Cominism in the other world.
I have read about this, this is a problem that exists even before wed novels, those who write lewd content have always been considered low status by writers of other genres, and they get upset when a lewd content writer is successful and popular
1. Ranma ½ - The first anime I saw, I loved it for its absurd comedy and its fanservice
2.Overlord - First isekai I saw, very fun, protagonist with questionable morality and good world based on D&D
3.JoJo's Bizarre Adventure - I love this saga because of how adsurd and epic its situations are...