I will say that I think you need to give Dressrosa a little more credit. It suffered all the pitfalls you mentioned, but I would give it some credit here and there.
Still, Part II has been mostly disappointing. The Fishman Island arc was horrendous, for one. It wasn't nearly as dragged out as Dressrosa, but the pacing was terrible. Sanji became a twat to force the plot forward. Chopper's idiocy broke the limit at the start of this arc. I don't like Usopp's new plant powers, they were introduced hastily and poorly and it just seems like an easy way to make him relevant later (of course they just make him a pussy all over again later on, just because Usopp). He's not the only one who seems to have regressed. The cast just seems simplified in general. Nami and Robin's tits are ridiculous. And although Robin and to a lesser extent Franky's new designs have grown on me, I still miss the older ones. I never liked Brook, but I like him even less now. The series has gotten more self-indulgent. The villain in Fishman Island was fuggin pussy and just overall sucked, as was the villain at Punk Hazard. Both still contain important plot points, moreso than Thriller Bark at least, but I still think that those two arcs are the worst OP has to offer.
As you said before, many characters just feel thrown in. CP-0 feels thrown in and was clearly never intended to exist. The Donquixote guys were underwhelming as hell. I don't really like Rebecca and feel like she got too much panel-time. That one whatshisface random guy (Chinjao or whatever the fuck) having Conqueror's Haki makes the fact that Luffy had it feel much less important, because this guy is a nobody, and it makes the ability feel less unique. The new admirals feel thrown in and I don't care about them. A lot of people feel thrown in and I don't care about them.
Still, Oda's continuity is still impressive, he's not entirely lost track of his plan as he still foreshadows/has shown us things that were foreshadowed in the past, so yeah. I expect One Piece to pick up somewhere down the line, but as of now I'm not too impressed. I would talk more about the positives, but post-TS OP has gotten me in a foul mood and I also have some things to do, so yeah.