I do remember some of your suggestions. Using certain words (TTrick already does this), but having a wider vocabulary to trigger certain things could work. The problem with that, it's not really obvious to the player that they will get something from posting certain comments. So I think the game itself has to hint towards that as well as what types of words do what.
You also mentioned making easter eggs easier to find based on MC's comments. Great idea. And then having those comments affect the girl's stats, as well.
I think most people are put off by having to individually click into each Insta post, comment and/or like. That's just really awful, which is why my Ren'Py port at least allows you to mark all posts as read and liked with one click. Nobody wants to sift through all those posts and do that, especially when trying to play multiple routes.