The point you made about Rey’s threat being her own insecurity and unsurety pushing her to the darkside is a great one! And I wish THAT had been made a little more concrete and was done a LOT more cleanly and effectively! And you can see how that’s sort of what they’re TRYING to do by “completing her arc” on Exogol, but it just doesn’t land because it’s all done so poorly, and it’s being sorta/kinda presented as PART of the same thing that leads to Kylo’s redemption, but the two aren’t really linked very strongly at all, and it ends up diminishing BOTH arcs in the end.
This reminds me that I saw something about a deleted moment where Ben tells Rey not to strike Palpatine down in anger on Exegol. I’m not sure how this scene worked in practice, obviously, but if true that it existed it’s a real shame it didn’t make it into the film. In my mind having Rey and Ben face essentially the same struggle seems redundant, but if there had been a moment like this, where you see Ben plead to Rey not to make the same mistakes he did, I wouldn’t have minded it as much. In a fitting way it would have been full circle for his “you need a teacher moment.” Not to mention it would potentially have resolved the issue where Rey seemingly does exactly what Palpatine wanted (killing him) and that’s a good thing somehow.