During Rey’s second lesson, there is a shot of Rey sitting down listening to Luke offscreen, and Luke says, “For many years, there was balance, and then I saw…” before cutting back to Luke saying “Ben.” You could possibly have Luke say something here like, “For many years, there was balance, and then I saw a dark vision of…” (you could perhaps shorten that line in some way if that was an issue). So within this scene, you learn that a dark vision of Ben’s future was the impetus for his training. It actually works well with the rest of the dialogue in that scene if you paint the picture of Luke knowing Ben’s dark potential from the get-go, but thought he could save Ben like he saved his father. That’s arguably close to canon, but we’re just fine tuning the details to recontextualize certain character choices.

I have other thoughts on the other flashbacks to the temple massacre and that event in general that I can share later. I do think you could potentially reuse shots from the various flashbacks to extend them and add more dialogue if needed, but I think a part of it is just adjusting word choices to get that idea across better in the same amount of time. Just depends on how detailed you want to be versus how minimally you want to change the existing dialogue.