When you actually grow up instead of staying a pseudo-nihilist teenager forever you start to cringe at crap like "King of Hell", which is literally what anybody trying to sound mature and dark will come up with for their generic fanfic, and you develop a taste to appreciate stories as what they intend to be and judge them according to how they actually manage such intention.
Again, the core tone of this story is like a bouncing rubber ball. It's about the main character being designed after a literal kid, about a power that evokes carefreeness and unpredictability, about reaching Laugh Tale to do what Joy Boy couldn't, about becoming the freest person alive... and now about bringing happiness to others with drum rythm. That's Luffy and that's One Piece first and foremost.
You are reading a story that is obviously more about making you feel like a kid and filling your heart with wholesomeness than any dark, pretentiously mature (not in actuality), uninspired crap you want to come up with, whether it's Zoro calling himself the king of hell or a fight based on black sparks over and over. As you well said, Zoro being edgy and cringe is for teenagers; hopefully one day you will actually mature and will understand that you were the real kid here, not those who don't pretend like Luffy turning the world into an inflatable house or behaving like rubber hose animation isn't what Oda's story has always been about; which is Luffy using a fighting technique from Tom and Jerry to battle the Government asssassin trying to kill his crewmates.
And of course there will be some seriousness; but you guys don't want seriousness, you want pretentiousness.