I wasn't terrible...it wasn't particularly good either unfortunately.
There is a lot of problems with it that I won't go into now, however, I will flex on how much I dislike the Phoenix.
The original concept, which was also the best concept IMO, was that Jean with Phoenix powers was just the full extent of her powers, she was a god basically but the power got out of control and she feared for what her life would be like with no control over her godhood and killed herself. Then they retconned it to be a space a avatar and things became extremely convoluted and it takes away from Jean's character as she would just be possessed by an evil demon basically or an alternate personality. This is what they went with in this film. They didn't even have the Phoenix speak through Jean or anything, it just apparently dredged up her memories and had her more angry than she normally would be, it was sort of riding the fence between the X3 Jean and space avatar Jean.
Nobody can ever do the Phoenix right I feel. Even the cartoons have had some weird takes on it. The Phoenix should just be Jean's full potential realized and as she realizes her godhood she becomes more distant and austere making her the antagonist. Inspirations I would point to are Luc Besson's "Lucy" or "Akira" when handling it.The route they always go down with it is like a bad soap opera.
I rewatched this a few times after release. Its implied that the force was unlocking Jean's full potential hence why her powers were getting stronger. There is a scene where Jessica Chastain says "You're special, Jean. You're stronger than you know".
I think I remember Simon Kinberg saying that the force "awakened" something in her back in 2017.
EDIT: Here it is.
“Jean absorbs those solar flares and then hurls them back out into space,” explains director Simon Kinberg of the sequence shown in the above exclusive concept art. “That’s what triggers, awakens, creates this Phoenix force and voice inside her.