Hi I am starting to read it, but if it seems linear so far... is that right?
It isn't very much likely, because there weren't any reasons to have a choice in the story right now. We have 4 romanceable options, so I'm sure we will have options to choose, not to mention that the story feels like its genre is mistery or thriller and these genres has a high potential for a non-linear story. So don't worry about it, it's not linear in my opinion, it's just the first build anyways. :D
If you're asking if there will be a branching narrative for all the friend, then yes if this follows the story. If you're asking if there will be choices, then yes again if this follows the story. I don't know if the VN will expand upon them, but I think there is about three to five choices once you're locked into a character. The choices leading to a "good" or "bad" end depending. Again, this is all speculation based off the story that might be getting revised or changed for the VN.
If this does follow the story, then the choices will only effect your relationship with the route character and not the narrative that route is telling beyond that. The story will still lead to the end of the route narrative, so long as you avoid a bad end. For an example, you learn about Joshua's history and experience some horror, but if you don't help him resolve his issues then the narrative can end abruptly. Trying to avoid specifics to not spoil, but the story I keep talking about can be found and read if you want to see it for yourself.
Granted, the story for three of the four characters are still in progress on Ao3, so that might change. Again, mostly basing my statements off of what was written in the stories.