"OK, maybe I was unclear here. To my mind, if each book is standalone, then they are
separate stories."
Right. I know. But separate stories doesn't make each book a different series. You have all sorts of stories within a single series, especially if the series is focused on a single character, like you seem to be planning. What I'm saying is, each book in a series is part of a cohesive story itself. Trying to write a 450k+ monster and then lobbing the marketing and publicity efforts on the publishing house...that's going to make it very very hard to sell in the first place, because, ask around. How many editors and agents want to even touch a 450k+ story, plus have to put all the work into figuring out what part goes where in the series, plus editing the whole thing, plus...
As for editors buying habits...Just because an editor buys your story doesn't mean they think it's perfect. Every story is going to need some work, and pretty much every story that gets published goes through several revisions before it hits the presses and experiences significant changes to the text. Things do change in that process, and if you want your series to be set in stone, its going to make it incredibly difficult to sell, because editors much more prefer working with an author who is willing to accept some criticism and work to make their story stronger based on feedback.
With things like taking out the sex, changing a character's hair or skin color..that's not what I'm talking about in affecting your series. What I'm talking about is...you've got a character who appears in four of your...however many books you end up with. They play a part in several key scenes. They're a romantic interest for the hero. Problem is, when you try to sell the story, nobody likes this character. They're flat, distracting, and uninteresting. No matter how hard you try, this character just doesn't come across well, and it would help the story if they were surgically removed. This happens, sometimes no matter how hard the writer tries to make it work, or how skilled they may be. But if you've committed four books to having this character around, you may well flaw the series.
I'm not trying to dissuade you from this series. Series are great. They build a career, a character, a story. But I think you are making this much more difficult on yourself than need be.
Lastly...as for finishing a book and then going back to revise previous books to make it fit better? Unless you have every detail mapped out from start to finish before you even start the first book, it's going to be practically impossible to get every book "perfect" within an entire series. You can't predict what may happen. What if you actually get tired of the this series or this character? (I know, you won't, but what if?)
The reality is, you're always going to be able to look back at something you've written and see a way you could've done it better. But if you spend all your time going back and revising, then that story will never get finished, will never be perfect, and will never have a chance to get shopped around because you won't ever be satisfied with it. Doesn't it make sense to take that first book, polish it as best you can so it's as attractive as it can be, and then start shopping it around while writing the second book? Trust me for knowing (I work at a big publishing house), but lobbing several tomes of writing at an editor's or agent's head is not an attractive way to get your writing accepted. Show you can at least write one book first. Then show you're working on the second one and aren't a one-book-wonder. But don't hamstring yourself by visions of perfection and constant revision.
Pardon for the length of this. Fingers wouldn't listen when the brain said stop.