Yeah, X-Men is definitely the most obvious predecessor here (even then, I would argue that Blade got there first, but X-Men is the first one in terms of "people knew it was a comic book to begin with", while people tend to think of Blade as just another vampire action movie). And then there's Daredevil (and, to a less remembered extent, Elektra). My point is less that Batman started the dark ~realistic~ superhero thing and more that the ease of slotting him into that kind of movie compared to other superheroes is the likely reason that we've had five major live-action Batman iterations (depending on how one measures the clusterfuck of inconsistent casting and direction that was the Burton/Schumacher films) while most others can only claim one or two.

I also agree that seeing what MCU got away with is the main reason DC decided to start getting a little weirder (coupled with the lukewarm reception of The Dark Knight Rises, which granted could never have met the expectations of its predecessor). I haven't seen Aquaman yet, but I'm still amazed it even exists given his reputation, especially given the points you brought up.

And I'm also with you on the "self-made hero" thing, which is usually brought up by the dudebros in question (ex. the "Batman is better than Superman because he does everything by himself without sidekicks or powers" speech I had to endure in my intro to communications class last year)

tldr you're basically right on all those points and I wish I wasn't still fuzzy brained from illness recovery so I could express it better