I've been told KU readers aren't going to be interested in an author who occasionally may put a book in KU. If they can't read the whole series, they will for the most part stay away. But if you're making a good income from the other outlets, it makes sense for you to stay wide, anyway.
I've had all my books in KDP Select at the same time before for a long time. In or out didn't make much of a difference.
I do not have a series (yet) though.
My last book (well, my last book was a book of poetry but I'm not counting that as I didn't expect any big sales for that at all) was a space opera. It was my second book that fit into an actual genre and probably the first that followed the tropes and expectations of its genre. It works as a standalone while also a first in series.
So, I figure, it fits in a genre, it matches genre expectations, etc., etc., so it should get a baseline of sales. And, even if 90% of KU readers won't read a book unless an entire series is available, there's got to be 10% or at least a reasonable percent that would try a first in series and/or standalone book.
As a disclaimer, I didn't market it as a first in series or anything, as I don't want to set up any expectations because if it didn't do well, there might not be any justification in following it up.
That's the one that made 29 cents in KU. (The previous one made squat in KU.)
So, I gotta figure that, okay, if the worst case is that only 1% of KU readers who are interested in space opera will read my standalone/potential-first-in-series book and I made 29 cents, then the other 99% who won't read it unless I have a series in KU means I am only missing out on $28.71 in sales. That $28.71 in sales wouldn't even cover the copyright registration for a second book.
If you want to count read-thru, then if you calculate a best wish scenario of maintaining 80% of your readers through a series, then the first book makes $29 and each subsequent book would make $23.20, then I'm still not even covering the copyright registration fees.