A major problem with all 3 movies was Rey’s constant winning and Kylo’s constant losing.
A hero and their journey is only as good as the villain and their antagonism.
Imagine if Obi-Wan had WON the very first lightsaber duel back in 1977.
That would’ve killed Vader’s character from the start.
Kylo’s loss at the end of TFA was the first crack. Then losing against Rey again in Snoke’s room. And then being humiliated by Luke. And then losing again to Rey again 1/2 of the way through Rise. Also, her ability to beat Luke while training didn’t help any character progression either.
Rey had no consistent hero’s journey. Yes, yes, you can make all sorts of arguments about scene x or development y, they’ve been done before–the point is that the growth of the heroic character overcoming obstacles isn’t coherent. Since Kylo was neutered as a threat midway through TLJ, they had to use Palpatine to give her a new challenge, which didn’t thematically fit at all from her starting point.