Palpatine changed his mind about using Rey for the Sith ritual because her parents abandoned her as if she was worthless because of her lineage/hid her away (yeah, you can still make Rey’s parents “bad” people, which would preserve Rey’s TLJ arc of learning to stop caring about her parents because they thought she was worthless) and he didn’t know she was on Jakku. Then Palpatine decides to go after Ben, the descendant of the Skywalkers. He demanded Ben kill Rey since he no longer has any use for her. When Ben redeems himself, this fucked up Palpatine’s plans to possess a healthy, non-clone body, so he goes after his own granddaughter again.
But Kylo was on Exegol previously; there was no reason Palpatine couldn’t have done it when Kylo first arrived, if Rey wasn’t needed for the ritual. I also don’t follow why Rey’s parents abandoning her would affect Palpatine’s plan (I agree it would be better if TROS stuck to TLJ’s arc of Rey-being-her-own-true-parent), nor Ben’s redemption, actually.
I couldn’t follow that final scene in the cinema and even now it’s a rubick’s cube to try and figure out in retrospect.
Palps believes that Kylo killing Rey would cement himself further into the dark side, for the ritual to work. And the reason Rey’s abandonment affects Palps’ plan is because he no longer knows where she is, therefore he is unable to find and get her, so he turns to Ben instead because he does know where he is.