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Some people don't consider it to be anime, but if you like subtlety, then you'll probably like Samurai Jack.

Nausicaa of the Valley of the Wind is universally regarded to be fantastic and sure doesn't have many of the annoying tropes of modern anime. It's from 1984. It's very heavily environmentalist and antiwar, and also just beautiful to look at. Drag's #1 recommendation.

If your problem is actually with the childishness of many anime and everything being too "kawaii", then try out JoJo's Bizarre Adventure: Phantom Blood. From episode 2 onwards, the main character is a grown adult built like Arnold Schwarzenegger in his prime. It's still exaggerated and frankly ridiculous, but the artstyle is mature and gothic in a way that might appeal to you. It's still a shonen (Designed for the sensibilities of male children), but it doesn't have annoying kids for main characters.

Kaiji: Ultimate Survivor is another example of a mature but ridiculous show. And it's ridiculous in a very grounded way. Kaiji is an unemployed loser who signs up for an illegal gambling tournament where the consequence for losing is literal slavery on the black market. No fantasy or science fiction themes, just a man past the brink of desperation trying to survive. The emotions are exaggerated compared to real life, but they feel incredibly grounded in the context of the show.

Drag tried watching Assassination Classroom and didn't like it, but maybe you will. It's a very slow burn mainly focusing on personal relationships between a class of students and their teacher. It would be far from drag's first recommendation, but maybe worth a try.

Drag would also like to suggest Kamen Rider even though drag is certain you won't like it. See, it's live action, but filmed very much like an anime. It's the Japanese equivalent of Power Rangers and is created purely to sell toys to children. You'll probably find it annoying, and experiencing that annoyance in a live action format might give you more clues as to what exactly you dislike. Kamen Rider Build is a good one to start with.