Michael Arndt, Simon Kinberg and Lawrence Kasdan had a writers' retreat at Skywalker Ranch in 2012 or earlier to break the story for the sequel trilogy. They came up with initial concepts and Kinberg claims they created a blueprint for Episodes 7-9. (Kinberg talks about that here: http://collider.com/star-wars-simon-kinberg-talks-writers-room-force-awakens-involvement/)
Then JJ Abrams was brought in last minute to replace Arndt and overhaul Episode 7 right before it was scheduled to shoot. Rian Johnson was brought in while JJ was still working Episode 7 to write the Last Jedi script. Rian said he saw plans - but that likely was what the initial story group came up with and JJ had tossed most of that out. Rian only wrote his script based on reading the Force Awakens script and seeing some footage which was filmed (not the full film).
Rian's producing partner, Ram Bergman, has said 90% of Rian's first draft made it to the screen. IMO, that's the biggest problem - they only started writing the script for Last Jedi when there was no time to actually make edits or changes and there was no time to have anyone plan out what was happening.
The story group at Lucasfilm helps maintain continuity, but don't make creative decisions.