I was curious myself, and here’s one breakdown by IMBD user ninewheels0 (link):


According to this count Rey has the most screentime in both movies, so calling hers (Kylo and Luke’s, really) the main plot is not wrong. However her screentime is, in fact, reduced in TLJ compared to TFA so your gut feeling is right as well. Kylo had slightly less screen time in TLJ than TFA, too, which is why his screentime count did not jump in comparison to other characters. You can see that he’s made a significant gain on Finn, though. He had 2/3 of Finn’s screentime in TFA, but the two are almost even in TLJ. And that’s just mechanically counting the minutes–I think we can agree that there was significantly more focus on Kylo’s character and background during the time he appeared on screen than there was on Finn during his appearances.
The forum post where I found the above count also did a comparison of how each character’s screentime in TLJ compared to their time in TFA (link). The result is pretty telling: Finn had the biggest negative shift in TFA-TLJ screen time other than Han, who died.

Finn getting the most time cut is in absolute minutes, not the proportion of screen time. Finn didn’t have the greatest proportion of his screentime cut among the living characters, but the only living characters who had a greater proportion of their screentime cut were secondary and tertiary characters such as BB-8, Chewbacca, and Maz.
RJ might not have carried through with his joke of keeping Finn in a coma for the entire movie, but he seems to have gone as close to it as he could get away with.