To me, interjecting something about Leia for Rey to react to with “wow, she still trained me though” feels weird. Makes it seem very elementary of a problem, like Rey is shocked that Leia was aware that she was having a hard time with self-confidence.
It’s not really “self-confidence” though. It’s that she has a natural inclination toward the dark side of the Force. Which is displayed in TLJ when the dark side cave calls out to her. It is definitely shocking to hear that your mentor knew you had a propensity to fail terribly but they still decided to take you on because they recognized you were strong enough to push through it all.
Chris Terrio describes it like the following: “We were very moved by the idea that Leia would have known that from the very beginning, but since she still saw such hope, heart and spirit in Rey, she decided that she was going to take a chance…”
Could you cut out “Because you’re a Palpatine”, and have Luke say, “Leia saw it, too” referring to the vision? So, Leia also had a vision of Rey turning to the dark side, but Leia saw her spirit/heart and trained her anyway? Or, keep the Palpatine line, and have Luke say, “Leia saw the vision, too.”