Judging the next chapters’ titles, I‘m not quite sure how much of it is just fillers, sounds like a good portion of them.
How so? Judging by the titles we should now return to the school trip mentioned in this chapter, which should certainly not be filler. Then some side couple stuff, which I wouldn't consider filler either, even if some readers don't seem to care for them. And again back to the school trip. The last chapter does probably follow the first again, judging by the title, but unlike the what if chapters of the previous volumes, I wouldn't really even consider these flash forwards filler. And with Yashiro's comments in this one, even those what if chapters became an important part connected to the actual story.
I‘m not sure, I mean this journey stuff could also imply the continuation of the future trip, the way they boarded the plane and it ends just there. But it‘s hard to tell when 4/6 chapters have the term "Journey" in there, it just sounds like it‘s all the same thing.
I hardly care about the side characters, to be blunt, I don‘t find any of them interesting enough to read about them, whether it’s the bonus chapters after each regular chapter or actual chapters dedicated to them. To me everything is a filler that doesn‘t serve plot, character information or development related to the actual story that is going on, which is AdaShima‘s relationship progression in their highschool years. It‘s already enough that they(NagaHino/Yashiro) have the last 20 pages of most chapters dedicated to them, they are hardly ever important to the progression of the story, when they aren‘t assisting the main characters in stuff that‘s related to one another right now. And about Yashiro... there‘s nothing really that I need to say about her.
But I guess I just make myself unpopular with this opinion.