Just one quick thought: I've seen people mention they were frustrated that Finn was sidelined in TLJ as their reason for disliking TLJ.
First, there's a difference between being *dissatisfied* with a character arc and *hating* a movie. Next, I didn't really feel like he was *sidelined* so much as...he had other shit to do. Mans was out there literally trying to help save the remains of the Rebellion who were getting run down like a Battlestar Galactica episode. Remember who Finn was in TFA: an escaped soldier/janitor who was scared to fight and genuinely afraid of the First Order. So much so that he was ready and willing to ABANDON REY (who he clearly was trying to start a thing with) rather than face them again. Even when he joined Rey on Starkiller and faced down Kylo, he was only there for Rey. Not for the Rebellion. Movie ends and he's in a coma and Rey is peacing out to find Luke.
Ya'll remember that, right? So if the next movie is going to pick up when that one left off, RJ has to do 2 things: (1) follow Rey on her own quest away from the rest of the cast and (2) take Finn on a quest that exorcises the bitchassness out of his system so that he can be in a position to grow into a true leader instead of one selfishly trying to save himself or the individuals he loves. After all, there were lots of people to whom his story could be an inspiration, who are dealing with the same kind of oppression and fear that need the same kind of help. And that's exactly what happens. Rey does her own thing and Finn is introduced to the perspective he needs to curve his fear into a fighter's anger. It's not refined as he's only just starting to channel that fear into fight. We see that as he almost gets himself killed at the very end. But he doesn't die and this puts him in great position for a 3rd film.
In fact, it's easy to imagine a 3rd film where the many lessons learned in E8 carry on to E9 and his feelings for the First Order are refined, allowing him to become a true general and the beating heart of the Resistance. Built on his passions, knowing better than anyone just how much there is to lose for everyone. They could have continued to build on his force sensitivity and -- perhaps in an alternative dimension -- we would see an Episode 9 where in some big climax battle, we get our suspicions confirmed as he force pulls the light saber from an injured Rey and goes to fucking work, showing off his quietly-kept light saber dueling skills (it is then revealed that Rey had been training him in secret). Then they both go on to save the day. Or galaxy, as it were. We could have seen Poe's newly-gained maturity and decision making in crucial space battles, earned after the costly losses of Leia and Holdo. We could have had Rose keeping the mains encouraged and engaged through rough situations and failures, ever reminding them why we can't quit. We could have enjoyed full Super Sayan Rey. Shit, maybe we even get the Rey x Finn a lot of us wanted to see. Mans took a light saber swing for her. Half of ya'll wouldn't even split a happy meal with a potential SO!
We could have gotten something like that. But we didn't because JJ decided that he'd rather not build off that all the good character development in E8, instead just turning the volume up on the action, taking a reasonable number of enemies and multiply it by 2 orders of magnitude for "coolness", relegating Finn's lines and presence to shouting "REY!" and running after her, riding a space horse, and limiting Rose to literally a background presence. Oh and returning Poe to a general state of childishness.
My ultimate point: TLJ was a character development movie designed to set up the 3rd act. It did plenty to develop the main characters through adversity. Every core character grew in meaningful, demonstrable ways from who there were at the beginning of the movie to who they were at the end. Nothing was taken away from the characters involved. They all got more complex and more interesting. All JJ had to do was take the new state of those characters (Poe - the best X-Wing pilot but now with real willingness to develop leadership skills \\ Finn - force sensitive and now with a true passion to fight for EVERYONE instead of just himself and Rey \\ Rey - now with inner peace/peace of mind of knowing who she is and that her lack of family does not define her future, the Jedi instruction manual in hand \\ Kylo now resolved to become the new emperor who must be stopped at all costs) and land the fucking plane.
It's not RJ's fault JJ couldn't land the plane. JJ couldn't have ask for a better set; just gotta be able to spike the ball. That's what he gets paid millions of dollars to do. If RJ had time to do E9, I suspect the trilogy goes down as the best of the franchise.