Honestly call it copium on my part but I actually do think Senpai will be endgame. Usually in manga, when the side character is being setup as the obvious loser, even if they get a lot of development the author will at least throw in some token development between the main girl and the MC.
But in this one, IMO, there's basically been nothing in terms of romantic development between main girl and MC. As you said basically all the romantic development in this one has been on MC and Senpai and the "will they evolve their relationship past the status quo" dynamic.
Like I dunno maybe I'm just hoping but it genuinely feels to me that this is et up for him and Senpai to get together at the end, Maybe that wasn't the author's plan from the get go but it sure feels like the plot has evolved towards that path.