I think it's a mistake to assume that all isekai need to be self insertion power fantasies. Grimgar is not a feel good story about some sniveling neet going on a hero's journey and becoming a God in another world with waifu hanging off both arms, but rather a rather disturbing and more realistic look at flawed people doing their best in crappy situations and failing as often as they succeed. If you really look at Grimgar this is not a hero's story at all, but rather the story of sympathetic villains selling their asses as mercenaries in a racial cleaning war to survive.
If your looking for a shounen power fantasy or simple black and white morality this is not the series you should be reading. Not all stories need to about the characters' growth as a human beings. There are are plenty of very well regarded stories about precisely the opposite. For instance Moby Dick is the story of a man destroying himself and everything he loved over his all consuming obsession with vengeance.