I think Kishi switched to NH some time after the Pain arc — probably about the same time he decided to extend the series into Boruto and the 'New Era Naruto Project.'
I feel like, up until the Pain arc, the series was going to end there. It was so good and cohesive. Unfortunately, I think the massive popularity made the series get extended. Kishi lost that first great editor, and the series drifted for a while. There were some good scenes, interesting story arcs, but on a whole I kept waiting for those individual plot threads to come together and make sense. It never really happened...they just then went into a war.
So at the time they decided to extend the series, they made the decision to launch Hinata into more of a lovey-dovey, worshipping girl to Naruto. Her character wasn't radically changed. She didn't become braver or play a more central role in the war. She didn't "shine." She just became more of a romantic lead, and a weak one at that.
Kishi says he planned it all along, but that's just a lie to justify the 'bait and switch' that took place the across the war arc. Because if he admitted he'd pulled one over on his audience, no one would stick around for the next series!! But it's still a lie, whether he admits it or not.
How do I know? Because it was a bombshell ending that Naruto wound up with Hinata. And it was only a bombshell because no one expected Naruto and Hinata to end up together. That relationship wasn't built up, and it wasn't supported by the character's actions or motivations.
It wouldn't have been a bombshell ending if Sakura had ended up with Naruto, as was built up the whole series. That was what everyone expected, the story was built around, and the characters evolved into wanting. There would have been no big, suspenseful ending to build a new series on.
So when they decided to carry on with a new series about their children, the switch was made to NH. Purely to satisfy/keep/gain new fans.
Another point to consider, if Hinata had been unpopular, would they have still paired Naruto with her? If Hinata had been less overtly pretty, less sympathetic, had she been more like an Anko, for example, would fans still have hung around to watch what happened with their children? No. They wouldn't.
If Hinata was not the sympathetic fan choice she never would have ended up with Naruto. There was no big 'planned from the beginning' sort of thing. No matter how many times they say it was there all along. Hinata was a plot device to move the story into the next gen. They determined that she was the female lead who would bring the most fans to the next series.
To me, the "True End" chapter is Kishimoto's true ending for the story. That's the one he planned out from the beginning stages. And that's why it's titled that way. Those are the true pairings and that's how he would have ended it. With their lives being saved by their two healer love interests, then going on to fight the ending battles and resolving the story. And there would have been no Boruto storyline.
But Hinata was needed for Boruto and the next gen. They needed a hook to bring fans over to the new series. And an NH ending provided just that.