Welcome.
I've just finished my first series, so take this with doses of salt - and also note that I was writing in a connected series, so a different position to you.
I wrote 2 1/2 books before I put the first one up, had book 2 on pre-order, then when book 2 was out, put the third on pre-order and so on.
The main reason(s) for me to do pre-order, in no particular order, were: I had something to tell my (very small) mailing list, and post on the facebook page; anyone who wanted the next book could see that it was on its way; and it also gave me a hard deadline to work to.
Amazon gives you a maximum 3 month pre-order and if you want to extend that, you lose the right to have a pre-order at all for a year afterward, so you need to be sure you can make your pre-order deadline. I was working on the book during the pre-order period, which was a bit stressful at times.
As you probably know, readers in KU can't pre-order, they can only read books that are live on the Amazon store, but I had a handful of sales on pre-order which meant each book had an actual rank and some also viewed / also boughts when it went live.
All that said, if your novel is ready to go, and it's a standalone, I suspect you may be better off just publishing.
Best of luck.