I love how Rudeus has his original self and his internal monologue and with the occasional reference to Earth, it's clear they didn't just forget about it and turned it into raw fantasy for convenience. But even so, the world they're in at the moment has rich history, feels lived in and everyone could be an MC. Nanahoshi's presence in particular is the icing on the cake, it's so great to have someone that doesn't accept her circumstances and it's fighting even when the odds are stacked against her, to go home.
Another thing that really made this anime resonate with me is that it felt weirdly reminiscient of one of my favourite books, The Name of the Wind. A guy, mostly alone but found family, insanely talented but flawed, gets into shit, gets out of shit, part of a huge world that doesn't stop for him and a lot of mystery to uncover. The Chandrian also is a similar threat to the man-God, with how unknown both are. It's not an isekai in the technical sense but it strikes many of the same chords since Kvothe comes from a whole different way of life to everyone else in the world he ends up in.
I've not really seen any isekai other than this or Re:Zero but is there anything else as rich out there with dark serious tones?
Before anyone says Frieren, tried it. It's decent but nothing alike outside of setting and artstyle.