PaulineMRoss
Inkling
There does seem to be a conveyor-belt automation assumption associated with the idea of mass producing books in the hopes that small sales figures for each book can multiply overall earnings to a desired level.
{Sorry to cherry-pick such a small part of your excellent post, but I just want to add a tiny comment here}
What I have found in my short time self-publishing is that the amplification factor for additional books is not linear. In other words, whatever a single book earns, a second book more than doubles it, a third book more than triples it. Here are my numbers so far:
1 book: $2/day
2 books: $20/day
3 books: $45/day
The 4th book is only just out, so it's too soon to tell, but I'd expect another jump in income. The reason is that each new book pulls in new readers who then go off and find all your other books. They feed off each other, in other words. This presupposes some connection between books. If each one is a different genre, this probably won't work, but it's why series are so successful. In my own case, they are loosely linked stand-alones.
Bottom line: additional books multiply earnings more than you might expect.