Going back and watching the first scene with Rey and Leia, I have trouble seeing where you could organically place that extra line.

I think we can assume this is some technique she read about in the Jedi texts, whether Luke wrote about it or some ancient Jedi. In the climax, you could possibly make it where she is hearing things the Jedi said in past films/shows/stories rather than them speaking directly to Rey. That would tie it in better with how the World Between Worlds was depicted in Rebels, if we assume the concept is connected. That could make it make the “voices” feel more like their collective memories being a part of the fabric of the Force, rather than them retaining some level of consciousness like the Force ghosts do. Something like that could fit in nicely with the “learn from the past” theme. I also had the idea of reframing Rey’s quick powers as her having the same Force psychometry ability that Cal Keaton and Quinlan Vos have, so this idea would expand on that one pretty nicely too.

Or, you could change it so she only hears lines from known Force ghosts, and the technique is how to connect with any Jedi who managed to retain their consciousness in the netherworld rather than just any past Jedi. Additional lines could be replaced with just unknown Jedi voices who may have managed to become Force ghosts in the past, rather than Jedi who we know couldn’t have become Force ghosts, like Kanan, Mace, Luminara.

Could also be a mixture of both, and make sure the only voices that directly addresss Rey are only known Force ghosts, with the other lines being potential past line “memories”.

I personally think what Jedi say during this scene should be overhauled, since what they say to her feels pretty generic. So yeah, maybe you could go with 1) Force ghosts only, or 2) past Jedi memories/voices, to help make it clear what is going on exactly. Or, a mix of both, with a risk of the waters still being muddy, but one that may stick closer to the theatrical version.