If you want to try the KDP Select option, it requires a 90 day commitment. I only do it for non-novel items to help promote my novels. My novels go up everywhere as soon as possible. Right now I am working on a short story collection that will feature the first few chapters of my upcoming new fantasy novel. It should have around 40k in 4 stories plus the chapters. I'm going to do one KDP Select 90 period before I release the novel and then keep on as long as it looks successful.
Keep in mind that sales can vary wildly from month to month for no apparent reason but I found that the benefits of participating in the Select program vary depending on the book. For example,
1) my novella sells for $1.99 with a 35% royalty. That means I make 70¢ for each sale, but the Select program typically pays close to $2.00 for each loan. In March, one borrow on the Select program paid me $2.10. I had to sell three books to make that amount.
2) One of my less popular books sells but rarely gets borrowed. On the other hand, my most popular book, during some months, gets borrowed more than the combined sales from all of my other books. In this case Select is Win/Win. Readers with Kindles and Prime membership get the book for free and I still get about the same royalty as I would from a sale.
As a result, I'm leaving the novella and my most popular title in the Select program but I pulled the less popular book from Select to repub it on B&N, Smashwords and Apple. (I don't do the other channels due tho their lack of responsiveness when I want to take a title off the market.)
I think you need to run some tests and see what works best for each title. Try Select for three months and then try publishing the same book elsewhere for three months to see how it does
As far as marketing goes, I think Robin is correct, the best promo is a good book followed by a better book. I also think that giving away the first in a series for free is a productive way to obtain needed exposure.