I think a clutter Earth can work really well under certain circumstances. We've had some individual great runs that have utilized the concept. But DC trying to have it both ways with its reboots (some of it happened, some of it didn't, or didn't happen the same way) makes it so their shared Earth, as a whole, has never really worked cohesively. Something like Young Justice escapes this because it's one narrative plotted out by Greg Weisman, planned as a clutter Earth from the beginning instead of haphazardly made into one from a Multiverse, and generally holds itself together better because of it.
I'm honestly not sure what the hell is going on with Wonder Woman now. But since she historically wasn't the first superhero in DC and hasn't been given that status very often, she doesn't lose anything by losing it. Superman WAS either the first or among the first historically, and is the granddaddy of the genre, so he loses something by losing it. Not to the point where he can't still work, but I feel like there's a trend of "an inch is given, a mile is taken" with a lot of Superman's stuff. "It's not important for Superman to be first" can get the ball rolling on "it's not important for Superman to be the physically strongest" and "it's not important that Superman's history and lore remain intact, so let's jettison the Legion," and we get the mess we get.
We'd have a Hawkman on the main Earth, it would just be Katar with no other crap piled onto him. Sounds pretty good honestly
The issues with Superman's rocket have a narration stating it was the dawn of the age of superheroes, which wouldn't make sense if it wasn't the actual beginning. Clark himself says "it's like the world is waking up" when other superhumans show up shortly after him. Sure he isn't around long before others show up...but in publication history, his invention prompted quick cash in attempts to chase the success and that's how we got other great stuff like Batman and Wonder Woman. He heralds the arrival of the age of superheroes. it has to start somewhere, why not with him?
It's not met with shrugs in-universe because he's the first to get the public's attention because as far as they know he's the first of his kind. The "first contact" doesn't lose its impact when its quickly followed up on by others. Meanwhile, his debut post-COIE might more logically be met with shrugs if everyone had seen superheroes already before he shows up.
The Phantom and Zorro aren't DC. Jor-L only debuted as an accessory of Superman's story. We literally have no Jor-L without Superman.
Superman definitely borrows stuff from earlier characters, but he was a unique combination of things that hadn't quite been seen before. He wasn't just influenced by one thing.