While pantsing is valid, I think that it's worth giving a shot at planning in Anon's case. I too am writing a super ambitious au, and it took me 9 months to thought-dump on-and-off, and another three to plan out the plot, characters, and world in detail. Yes, I am on the extreme side of things, but here's some wisdom I can share.
First of all, I feel like planning gets a bad rap for being "homework". Like I've heard "Oh, it stifles creativity" or "It feels like the story has already been told so why write it?" and I just... don't get that at all?
Sometimes planning doesn't work for you, and that's okay, but planning is an intensely creative process! This is where you lay the skeleton of your story down, and then actually writing it is where you lay on the flesh. Along the way, you may have to rearrange a few bones here and there, but you see a finishing point ahead.
Having a general path to follow is what gets be through longfics. I can't stand to run in blind. I need to see the gist of what's ahead of me. To some people, they feel outlining the story means there's no point in telling it because it's "already been told" but to me it's like inhaling a binge-worthy book at lightning speed and then writing the fic is like rereading the story to pick up on countless details, clues, and foreshadowing that flew over my head the first time. Even with my meticulous planning, the drafting process continues to delight and surprise all the time. It just comes from the details rather than the trajectory of the story.
When it comes to planning a large fic, though, it helps to get visual. Go beyond the typical Word or notes document. I am once again quite extreme because I pay like $12 a month for Milanote, but I use it because it allows me to easily navigate between my notes, colour code them, add visuals, as well as put in a ton of detail while also being able to see the bigger picture. Look into using a free program that allows you to do the same thing, or spice up your document with bookmark links, coloured text, and pictures.
When it comes to the actual content, these are the three places to focus on:
Plotting is about ensuring that you have a strong throughline. You don't want to get side-tracked and you want to keep the pace. Where does the tension peak and rest? Planning will help you find that balance early on without having to do a ton of deleting or rewriting.
Character profiles will help you nail down how they've been shaped by this world, as well as consistency. The thing to focus on here are goals, motivations, values, and development. That's the real meat and potatoes. If you know that deeply, then it'll be easy to keep your characters in-character.
Worldbuilding isn't my favourite thing (and I like that fanfic lets me gloss over it most of the time) but this is where you focus on plot-relevant regions, magic systems, cultures, and political systems. This is also where pictures come in handy. Instead of writing tons of notes, just get some inspo pics, add a few sentences below it for context, and vola! Worldbuilding!
After you've finished your planning, read through all your notes again. See if there are any changes, big or small, that you want to make before you start. It'll save you a ton of hassle.
And after all that planning, your longfic should hopefully look a lot less intimidating because you know exactly what you're getting into. There's no running in blind because the path is clear and now you only have to pave it.
