Yeah, ok. I didn't read the entire thread, because it seemed much of it sprawled into publishing and promotions and career questions, which is like discussing the frosting for a cake that doesn't yet exist.
From what I did read, it seems Slow Road wants to write books but has never actually written one. I hear this pretty often, maybe dozens of times. So when I tell you that no one who's asked me how to start writing has never actually written that book they have an idea for, understand that I have a decent sample size to work from.
For your situation as a late-stage FIRE-seeker, I'm going to use an analogy that tries to make a point. In college, there would be Friday nights where I would be up in my dorm room studying for a biochemistry test. It was not enjoyable, as you can imagine. I could hear people out having fun, and I would fantasize about all the neat stuff I could be doing when this damn test was over and done with. Invariably though, the test would come and go, and afterwards I never did any of that stuff I dreamed of doing.
The same thing happened with FIRE for me. I had these dreams about what I would be doing "after". Then it happened, and what emerged wasn't quite what I'd imagined. I played less golf, not more, and I got into woodworking, which I'd never even thought about. But writing made the cut, because I was already a writer. I'd written books before I retired. I had specific ideas. most importantly, I was excited about doing.
The only way to be a writer is to write. Your biggest problem, from where I sit, is that you don't know that you enjoy writing. You just think you would.
That's not a knock. I suspect you enjoy reading as well, but the difference between reading and writing is like enjoying watching a movie and working on a movie. Related, kinda, but far from identical.
Advice on getting started? Just write, baby. I blogged while I was working, here and on my personal website. I wrote shorter nonfiction stories (again, here and on my own website). Eventually I wrote a book. Then another, and another, and another.
For a full book, the easiest most effective way I know is to write a minimum number of words every day. Commit to writing 500 or 1,000 words a day minimum, no matter what. Even if it's just crap filler that you'll have to fix later, put down the words. Do that for three to six months, and you should have enough verbiage for a rough draft of a novel. Easy to say, not so easy to do.
Good luck, whatever path you wind up taking.