I feel it's a bit disingenuous to label her the final villain when she exist between 680-690. Also, just ignoring Madara for no reason; I don't know if you're just trying to give Oda more points by comparision. I'm too much of a Narutard to let it slide.
I don't think Imu has "hardcore prep". They were introduced as a silhouette, then the "empty throne" concept was introduced 2 chapters later. That's not very organic. For organic foreshadowing, how about we look at Madara Uchiha, naruto's penultimate villain.
In chapter 225, Itachi just drops this line and leaves.
In 234 we get a bit of context, but they never say "this is the guy Itachi was talking about."
(Side note: If I was as generous as an average one piece enjoyer. I would call this intentional pre-planned god level Ashura/Indra foreshadowing. )
Chapter 309 we finally get a name drop.
(maybe it is time to start considering this ashura/indra stuff)
Then itachi reveals that he's alive we know the rest. This shyt is organic, slow, and the payoff was great.
He comes back in the final arc at chapter 559 and rocks the arc until chapter 679. You can't just prop up a 10 chapter villain and downplay a 119 chapter one.