Glad to see you be more critical to one piece, to me it seems obvious that there has been a large shift of direction at some point - I think the Gorosei going from real-politik buerocrats, that do horrible things through tears for "the greater good", to "hahah humans are insects haha, we are literal demon monsters" is the best evidence.
The problem with the tonal shift is that Oda obviously wanted/planned to go through a shift into darker territory starting with the last part of Paradise, and then continuing into the New world.
He opened up with Luffy, sanji and zoro murdering pacifistas in rather graphic ways, the fake straw-hats being buried alive, Caribou's crew seemingly all being killed when their air bubble burst, with plans to delve into the destroyed Shogunate of Wano, Big Mom's horrific dystopia, have the kingdom of Dressrosa go through the bird cage, explore child experiments at Punk Hazard, and have the driving force for most of the New World be Luffy signing his name and crew unto several assassination plans regarding Kaido and Big Mom to take out his biggest rivals for the title of Pirate King. Nothing heroic about that.
On paper, all of that sounds like it would lead to a second part that though comedic, took itself way more seriously than part 1, and led to a comparitively darker follow up.
Of course, anyone who has read One Piece knows this did not happen. Oda ultimately didnt have it in him as a writer to go down this path he had set for himself, and the tone set early into the second half was quickly forgotten, and One Piece single biggest flaw, Oda's complete inability to ever kill off a character unless he absolutely, positively has to, returned and worse than ever, castrating the effectiveness of almost all of the part 2 arcs in one way or another.
The ultimate result is that part 2 overall actually feels way lighter in tone than part 1 ever did, with absolutely nothing part 2 ever did coming close to the horrors of the Alabasta civil war, the Marineford war and the torture palace of Impel Down.
Which puts the final part of the series in a bit of a mess regarding it's tone.
Because Playtime...
Is over.
Oda has now finally reached the point where he now HAS to write One Piece as he originally envisioned it, with the dark tone, and brutal battles he originally had in mind, where characters die, because he actually needs to remove them from the board now that we are here at the finish line.
The problem is the fact that it's so fucking jarring.
Also he is realising he doesnt have the time left to let things play out in a slow burn as he has done all through part 2.
I have no doubt that Oda originally had some plan in mind where the Gorosei would act as the usual Miniboss squad of One Piece's villains and would be fleshed out over time... But the simple, brutal fact is, he doesnt have the time left.
Oda is both Old and tired, and does't have the energy he had 10-15 years ago, but even if he had, he doesnt have the arcs left where he could take the time to introduce them one by one.
I honestly thought that was what he had in mind for Egghead, but then suddenly he realised halfway through that there wasn't gonna be any arc to do this after it. So he had to basically cram everything about them into the arc now, now, now, and he was forced to saddle all of them with the one, single fleshed out elder he had planned, so now they all have god complexes, when that probably was only supposed to be spider guy's thing.
Hey, remember that there was one of the Gorosei(The boar guy)that objected to just wiping one of their kingdoms from the face of the world for a test drive before falling into line when push came to show?
You probably dont, because it's obvious whatever that was supposed to be setting up that one of them had scrouples seems to have been abandoned/forgotten.
There are a lot of Problems with part 2, but basically most of them boils down to the fact Oda's editors didnt force him to follow through on the tonal shift, and didn't reign him in when the arcs began to baloon, and that spiraled until we ended up here at the end when everything is now for all the marbles.