spoilers:end of spoilers
1) He literally commanded the entire ground troops, from Rose telling her to go to handling the entire defectors of first order troopers.
2) Just because she found Lando does not mean her interactions with Finn comes to an end. He not has an entire community based on the same foundation from where he came from, which can explore untold amount of potential.
3) Finn feeling that Rey died confirms he's force sensitive, it's a perfect example of show, not tell which is the foundation of writing. They continued to give hints throughout the film which culminated when Finn felt Rey's death.
In TLJ, his "character arc" was a poorly executed rehash from his arc in TFA. he didn't even give the defeat to his nemesis Phasma, it was circumstances that took her away, not him. He wasn't successful in almost a single aspect he performed during the movie.
So we'll have to agree to disagree on what a character arc is if you don't think all of those changes to his character and foundation doesn't constitute a character arc.
Ultimately, I have no problem with people preferring TLJ over RoS. We all have our interest. My issue came from your original post calling out folks who like RoS over TLJ.
Was RoS a perfect movie? Far from it, the fact the writer asked for 2 movies kinda cemented some of the main problems I had with the film. (It truly needs 2 movies or at least an extended version like lord of the rings), but out of the trilogy, it was definitely my favorite.