I wouldn’t consider a Marvel movie particularly “adult,” no. Anyway, every book written for an adult audience, over literally thousands of years, is “dry and boring”? At a certain point you need to look in the mirror.
I'm sincerely hoping to be proven wrong.
If you want to try going in a totally different direction, I am a big fan of Abe Kobo's avant-garde work, such as The Box Man or the Ark Sakura. I guarantee it's unlike anything else you've read if you've avoided straying from the kiddie pool, playing with the whole structure of the novel itself and challenging your expectations constantly.
If you want to try some nonfiction, I have rarely read a Paul Theroux travel book I was bored by and I think Ryszard Kapuscinski is also great.
I do think anyone can enjoy Dune. Although it's still essentially about coming of age, it has a compelling plot and universe, if you're into that sort of thing.
But compare with one of my fave books, Murakami's The Wind-Up Bird Chronicle. Main character is a middle aged guy whose life essentially starts unravelling. And it's like.. if you haven't yet gotten to that point in life where you can empathise properly with the characters and your sense of meaning of your own existence starts unravelling, it's not really going to make as much sense and maybe won't be quite as interesting or meaningful I guess?
A bunch of crazy shit happens, sure, but not of the pew pew lasers and kicking people in the teeth kind. I do think I would've enjoyed it a lot as a YA but maybe not as much. And the level of writing is different, Murakami (as translated) writes very simply but it's much more complex in terms of the underlying concepts and it's not like you stop getting better at understanding shit once you hit 25 years old.
So that's at least one illustrative example!