Thanks @TheoneandObi and @Lazysummersundayafternoon!
I wrote my first draft of the first book in about 45 days. The less said about some of the scenes the better, and as for the character introductions - eugh.
But the fundamental flow of the plot is the same now, even though radical edits have been made to the plot.
I keep a separate side doc with notes in chapter order, and if there's something irrelevant that needs to be right (like a date or eye colour) I just leave empty brackets to fill in later.
It's very alien to me to think of trying to write perfectly first go! I don't even write in order. I started with a chapter from the middle of the third book. I write whatever character or scene is speaking to me on any given day. It means mostly writing starting points and end points for each character, and filling in getting from one to the other.
Means some awful writing, but for me it's a real shortcut to the good stuff, because I never get "stuck" - I'm always writing something I feel like writing. So I never edit until I'm done.
First edit is to work out any big plot problems - read through, does it all make sense? Is some crucial bit of information missing? Do all the plot bits line up? I correct those things first so that the next phases of editing are based on the right info.
Second edit for me is about storytelling and the writing - taking out the horrible cringe stuff and writing chapters as well as I can. This is the biggest job, as some will be crap and need rewriting. Sometimes even piss poor chapters can be wholly rewritten very quickly once you know what you're doing with them.
Third/fourth are about making it briefer, more concise and perfecting the SPAG.