I'm not even entirely sure you could do it as a movie series at all, at least not to have it be emotionally satisfying. X-Men: TAS knew to do it in two chunks (although even that was missing an episode that was supposed to go between the two, which ended up airing way out of order), but they'd also been building up their characters for two years before they tried. I don't think there's a way to do that even in multiple movies, not when they have to juggle so many characters. You'd need at least one movie that's primarily ABOUT Jean before getting possessed by the Phoenix Force (maybe that's the climax of that movie) and one about her as Good Phoenix before you can bring it home in any meaningful way. And Fox, at least, was never going to focus on a character who's name doesn't rhyme with Schmulvarine.
It doesn't help that the only character who was ignored more than Jean in both X-Men trilogies was Cyclops.