One problem is that Amazon is one of the worst digital sites on the planet when it comes to providing any actual data to analyze how your product is doing. Currently there is no way to track how many visitors see your books, what keywords led them there, what is the conversion rate for sales/borrows etc.
There is no way to even tell when exactly the keyword changes really "go live". And because there are up to 7 keywords, you might change some for the better, some for the worse and be no wiser. Maddening.
Dealing with promoting your stuff on Amazon and trying to narrow down the best methods, is like trying to find a needle in a haystack, in pitch black darkness.
With all that said, I've done my share of internet marketing and keyword research and I own a couple of premium Keyword research tools. I can use these tools to see what keywords are most popular and with manual research I can figure out fairly optimal genres and keyword combos for any given book. In theory. (Of-course, success hinges more on a good cover, title, blurb etc. than it does on keywords. Keywords can make the book more visible, but that doesn't guarantee more sales.)
I'm willing to offer my help in researching the best keywords for a book. I'm not going to ask for any money for this as I have no data at the moment to suggest if my methods will be successful.
If anyone wants to experiment with me, send me a PM.