When Rey has her vision during training, she sees flashes of the battle, her friends in danger, and herself on the Sith throne. Maybe this isn’t necessary, but you could add a shot of Kylo somewhere there with her. (I think you could potentially edit that BTS shot of Kylo in his chambers and make it look like he is on Exegol. Or maybe a different shot of Kylo on Exegol could be used.)
Kylo tells Rey that if the Resistance goes to Exegol, they will all die. She will fail. Again, Palpatine could kill Rey and the Resistance on his own, using his fleet and followers. He just wants Kylo to prove that he has fully embraced the Sith path (or just the dark side) by killing Rey, his “light”.
The flashbacks during the Force bond duel would be replaced with “flashforwards”. Swap the first flashback with shots of the Resistance/Finn/Poe losing the battle, and the second flashback with Finn and Poe “dying”.
Then, the hangar revelation would be something like this.
Kylo: Rey, I know the rest of (y)our story.
Rey: Tell me.
Kylo: I saw what we will become. We won’t just have power. We’ll have his power. We will kill Palpatine, and take the throne. I feel the pull to the light, and you, the darkness. What Palpatine doesn’t know is that we’re a Dyad in the Force, Rey. Two that are one. Together, nothing will stand in our way. You know what you need to do. You know.
Just to point out, I don’t think Kylo sees this as him taking the Sith mantle. He sees it as taking their power away and using it for his own new order. In the end, though, his opinion that he is different than the Sith might just be misguided. But it is important for him to say that since we see Rey on the Throne in the vision.
The one thing here though is - why would Kylo want “his power”? Why would he want to take the Sith Throne, when his mission statement at the end of TLJ was to let the past die, including the Jedi and the Sith? I almost feel like he should be an anti-hero - wanting to stop Palpatine in order to destroy the Sith, so that he can take control and make something new. (Ha, “I will finish what you started”…by destroying Palpatine.)
In my mind, I think the conversation should go something like this:
Kylo: Rey, I know the rest of our story.
Rey: Tell me.
Kylo: I feel the pull to the light, and you, the darkness. I saw what we will become - a Dyad in the Force. Two that are one. Embrace the Dark Side, Rey. Join me. Together, we can stop Palpatine. Nothing will stand in our way. You know what you need to do. You know.
I also kind of like this, because it helps to explain the Dyad stuff a little more. They didn’t just become a Dyad off screen, they need to unite in order for it to happen, and then they can level up. Kylo thinks they need to be on the Dark Side to be a Dyad. But in reality, it will happen when they stand together against Palpatine on the Light Side, culminating that arc. That then explains why Palpatine didn’t have any clue about it until the end, and didn’t factor it into his plans.
As for the Quarter scene, do we even need flashbacks or flashforwards? Would it be possible to edit it in such a way that it focuses on just Rey and Kylo? Don’t get me wrong, I think “If you try to defeat him on your own as a Jedi, you won’t be strong enough, and everyone you care about will die” is a really good way to take the story. (And it sets up for Palpatine’s later temptation, rather than throwing that into the climax out of nowhere.) I’m just having trouble picturing how to convey it effectively. Like, if you want to show Poe dying, you could have a shot of him being rattled around in his X-Wing, and then cut to Greg Grunberg crashing into the Star Destroyer; but that sort of manipulation would be obvious when watching the final battle itself.
God, my brain hurts. This is what happens when the original cut tries to implement 18 different contradictory story concepts all at once, and we’re left trying to filter through them.