If we put back Palpatine’s line, “I never wanted you dead, I wanted you here,” it would not be clear that he changed his plans. Originally he really did want her alive all along, even when she was a child. The narrative for Ascendant is that he really was trying to have her killed, since he had already invested years and years into Kylo Ren with that mighty Skywalker blood.
This is the way to do it. Canon is that the Skywalkers have more potential strength than Palpatine, so it does make sense. Of course, that particular line is removed from Hal’s edit of ROTS “[Anakin] will become stronger than either of us.” But I think it still stands fact regardless.
So Kylo is stronger than Rey (he easily beats her on the Death Star wreckage). And Rey would have been born when Ben was around 9 years old. So it makes sense that Palpatine always wanted to kill her. Maybe in this continuity he never meant for his son to escape Exegol.