The foundational structural issue lies with the fact that the duel on Mortis has f@ck all to do with anything happening on coruscant
This undermines the story
It needs restructuring but has a perfect plotline otherwise
The story is about the heroes using the force beacon to incite a revolution against the first order…that is unique already
The structural issues however are a big problem
We need a plot device that gets kylo on coruscant to fight rey on coruscant in a battle that centers on kylo and his plans.
This could have been solved with stealing the climax of fullmetal alchemist and having coruscant be the center of an alchemical ritual meant to absorb all life in the galaxy and turn kylo ren into a living god, a ritual he discovered by studying the holocron of darth plagueis(snoke) And he needs to learn force draining from Tor Valum in order to learn how to absorb so much godly power…putting Rey and kylo in the middle of the battle of coruscant and adding bigger stakes to the story that is befitting of the final Skywalker movie. To connect the stories better, you could have each of the millions of planets in the galaxy have 36 obelisks on them that is supposed to absorb the life force of each planet and than beam that life force energy to coruscant and specifically the citadel structure that is supposed to in turn beam it to kylo and turn him into a God. The beacon under the temple will send a telepathic transmission to every living sentient being in the galaxy to warn them of this ritual and organize an uprising to take down the obelisks before kylo consumes all in this mad quest for godhood and immortality, this could also be what kylo believes darth Vader had started who also desired infinite power to attain immortality.
This would have formed a strong foundation for episode 9
Tor valum could easily be a flashback to the past, where kylo learns of the force draining technique and decides to become a god by absorbing the entire galaxy.