Order of Operations

So I'm in the middle of first-drafting a sci fi series, and I'm pretty sure it's going to end up being five or six books.  Each book is a romance, so each book features a different couple.  Some of the characters are secondary characters introduced in earlier books, and some of them are entirely new, and each book is complete, in terms of the romance.  There is a definite over-arching sci fi plotline that moves forward with each book, but I don't intend to leave any of the books on a cliff-hanger ending (because as a reader, I really hate having to wait months or even years for the next installment). 

And what I'm trying to figure out is whether or not it's best to first draft the entire series before trying to revise anything, or if I should be trying to draft a book and rewrite/edit the previous book at the same time, or if I should work on one book at a time, alternating between first drafting and rewriting. 

One thing I do know is that I can't do a first draft of one thing and a second draft of something else at the same time.  I tend to get so focused on the first draft that it's really hard to switch gears.  And if the two stories are in the same universe, it's hard to pull my head completely out of one and focus on the other, and keep all my facts and histories straight.

So I really think I should do one thing at a time, and my gut feeling is that I probably ought to first draft the whole dang thing before I invest too much time in revising, because I might come up with some fantastic idea for book 5 that requires some groundwork to be laid in book 1.

Unfortunately, the part of my Brain that tries (with varying degrees of success) to keep me organized and on track is now looking down at me with those ridiculous glasses perched precariously at the tip of its nose, and I'm trying really hard not to laugh as it asks me if this is just another example of Creative Procrastination.  Because, you know, if I don't ever finish anything, I don't have to submit anything, thus avoiding rejection, devastation, and feelings of inadequacy.

"Why, no," I say earnestly.  "If this was Creative Procrastination, I would be researching the mating habits of the stick insect, or hopping in the car for a trip to my favorite yarn shop, or scrubbing the toilet with a toothbrush (not mine, of course), or scouring the mall in search of Shiny Things with which to decorate The Swamp."

Neither of us is really satisfied with this answer, but I suppose time will tell.  For now, as long as the creative fire is burning so brightly, I will dig in and take the first drafts as far as I can.  Then, we shall see. 

The good thing about this venture is that I'm not actually under any sort of financial pressure—Husband Beast is doing well enough that we are able to pay the bills and there's even enough left over for pizza and beer now and then.  And books.  There must always be books.