> Over the last couple months, I've been building world bibles, writing and visual style guides, and other documents for this project [...] about two weeks of additional polish work to cut out a lot of fluff and a lot of the LLM-isms.
The amount of work and walltime expended sounds about right. You have discovered / stumbled upon the relatively well known but little appreciated job of a publishing editor. It takes a lot of nitty-gritty work and built up domain knowledge ("world bibles") to direct a piece of writing - and its author - to a level where you confidently believe that you have captured the intent and desired tone of the piece, while keeping it sufficiently tight, engaging and interesting / non-patronising enough for its audience.
Disclosure: did ~decade of freelance writing around the turn of the millennium, and have had the privilege of being schooled by a small group of good old-school journalists. And then had a publishing editor assigned for a separate project, from whom I learned even more about writing.