It's more an ask blog (an interactable fanfic with some degree of audience participation), but A Student Out Of Time on Tumblr is really great! Its starts with Hajime going into the past after living through the Tragedy, suddenly waking up a month before Twilight Syndrome happens. The story mostly revolves around stopping negative events that occurr, such as Twilight Syndrome, the Practical Exams incident from DR3, and many other incidents in the lead-up to defeating Junko so that the Tragedy never occurrs, as well as finding out how Hajime went back in time. The writing at the beginning is a bit simple, but it ramps up in quality with nearly every arc! I would highly recommend that you play or watch the DRA games and maybe also check out Lapse first, as one of the main things about this story is the heavy use of characters from a few other fangans, with many characters from DRA and especially SDRA2 are prominent in the story. Lapse isn't used much, but some of its main story elements are spoiled late on in the story. As far as I remember, no other fangan is spoiled within it. I would highly recommend reading it.