To me, the solution would have been simple.
When Rey goes to Ahch-to, she’s avoiding her fears, but she should have also been having a crisis of faith in the power of the Force (which Luke helps restore by lifting the X-wing).
At the end, to complete her journey, she decides not to avoid her fear, but also not to try to use brute force to overcome it. As Luke should have done in the cave in TESB, she doesn’t use her saber, and puts her trust solely in the force. When Palpatine starts shooting her with lightning, she turns off her saber and lets the lightning strike her - but it doesn’t, her faith is rewarded and it phases through her, Luke on Crait style (but without the need for astral projection). The lightning going through Rey destroys the temple and kills Palpatine.
People would have hated it, but it would’ve been pretty cool and a fitting conclusion.
Eh, feels too much like a deus ex machina for me. I’d prefer for Force powers to have some sense of realism, rather than just being a get-out-of-jail-free card for true believers. Plus, if you can’t get hit by lightning when you trust yourself in the light side, why did Luke get hit by lightning in ROTJ?
As for Finn, instead of making the ‘I’m staying behind’ sacrifice move to destroy the star destroyers, the star destroyers should’ve all been goners, but he stays behind to hijack their comms and send a message to all the stormtroopers that the war is lost and they should abandon their posts and save their lives rather than go down with their destroyers. Something like that.
I tried to do a more extreme version of that in the rewrite I just posted. Finn doesn’t sacrifice himself, and the fleet doesn’t get blown up. Instead, he persuades the Sith cultists to rebel against their master, using the fleet to destroy Palpatine. I think that’s the most fulfilling possible evolution of Finn’s arc.