I heartily agree with your perception that the time to strart building an audience for your book is before it is finished.
When in 1965 Scribners first published “Miss MacIntosh, My Darling” by Marguerite Young the book was already famous -over its 17 year + period of composition it had gained an epic reputation among the literary elite – It was reviewed by the New York Times, hailed as the greatest single American literary achievement since Moby Dick, had a fair to middling sale, was remaindered in the Village Book Stores, was picked up by Harcourt for paperback) and now holds a high place in the literary firmament, with few readers on board at any given time, to be sure.
Well, that’s literature.
I have begun to put bits and pieces of my novel out as blogposts – “scenes” –
The characters become real in the reading. A reader responded she liked the speech of Ah Toy – so of course I will post more speeches by that lady, for that lady. Now, if you can gather online a few (let us say a mere thousand LOL) readers all of whom are clamoring for Ah Toy’s next speech, you’d be “on your way”- you’d have taken your first step.
There you are.