I'm writing a series (universe?) of books that tell the story of re-shaping the world for magic starting in 1973, coming forward to some point in the near future. (My MC starts from that point and is rejuvenated and sent back.) I'm on my fourth book, pushing 400,000 words and I've yet to cover two months. (The pace won't stay the same; there'll be some periods of relative stability. I think. Maybe.)
Anyway, these stories are developing a cast of characters, currently over 50 (not all yet in a story). Plus, there are key aspects of the emergence of magic I need to track. I end up asking myself, "How did I decide to handle that? What chapter was it in?" I'm finding that plotting each successive book is getting harder. I think it will get easier as aspects of the story stabilize, but, right now, there's a lot going on.
I've got notes in Scrivener with character sheets (in Scrivener), thoughts about the development of magic, a magical society, etc., descriptions of various character arcs, etc. I've been able to work with that up until this point, but it's getting tougher. Find myself looking back for "what did I do with this aspect of magic in that scene where...?"
I've tried a spreadsheet, but I'm not good with those for data management, so I'm tinkering with an Access database with tables for characters and chapters. I may add locations. I also track POV character and characters in each chapter.
Anyway, for those who've wrestled with this kind of stuff, what helped?