While I have some big problems with TLJ, I’ve complied a list of small changes that would be super easy to edit but go a long way. I only wrote down ones that haven’t been suggested yet (to my knowledge).

Remove Rey’s parentage reveal. This had been set up so well in TFA, it should has been saved for a dramatic moment. As of now the answer just gets blurted out by Kylo after so much buildup, not to mention the reveal is kinda disappointing. By pushing off the parent reveal until Ep 9 (which could be added in as a dream or into another Rey/Kylo confrontation) the film would feel less anti-TFA, keep Rey interesting, and give audiences a reason to see 9.
Remove Captain Phasma. Honest to god what did Starkiller Base accomplish? How did everyone get off in time? Phasma is shoved into this movie for no reason and then she dies (supposedly). Cut the fight and escape (we can think Finn+Rose died with the lightspeed attack, and then we will feel the relief Poe feels). As for the shots before with her, recolor her red stripe blue and put a voice filter / dub to imply this is another, male first order captain. A line with “captain ____” could help.
Leia is sick. Leia’s first “death” on the bridge is brilliant. Kylo doesn’t pull the trigger, but events have spiraled out of his control and he has no choice. You don’t play with the dark side, the dark sides plays with you. Now I’m not a fan of her return, but she is instrumental to the story plus the scene with Luke is great. How about instead the bridge attack severely injures the general and she only has a couple days to live. This could be enforced by a dubbed medic and Leia coughing offscreen moving forward. The resistance’s situation would be even more dire, Leia would be written off for 9, and Kylo’s attack now has a significant meaning.
Replace Canto Bight with Tatooine. Before you start screaming let me explain. After the galaxy learned that Luke Skywalker, the last Jedi who defeated the Emperor and Vader, had humble beginnings on Tatooine tourists began to flock there. In the wake of Jabba’s death casinos started up and eventually an artificial ocean was created. After 30 years Tatooine has become unrecognizable, with the entire planet having switched to the tourism industry. This fits perfectly into the theme of “Luke as a legend” and “letting the past die”, and has some real world ties to Orlando. Plus it gives us another (but new) look at the desert planet, which is essential to the Skywalker Saga. Lastly the final scene has a whole new meaning- as Luke dies a new child looks up into the Tatooine sky in hopes to someday become a great Jedi. This would be super easy to edit- Canto Bight is actually the name of the city, the planet is Cantonica. So maybe a loudspeaker could announce “Welcome to Canto Bight, the jewel of Tatooine”.

These changes, plus trimming some scenes to cut on the runtime, would solve a majority of my problems with the film.