Here's one sample keyword:

billionaire boss romance erotica obsession pregnancy forbidden series bareback menage mfm with mm a trois m f mmf biker kindle taboo sex bundles

That's 144 chars of the 400 allowed. That should give you an idea. It's the most basic approach, typing in your original keywords in various combinations and noting the suggestions. I have no idea what your book is about but if there is a specific kink or situation that readers might be interested then you want to cover all possible variations. I'd put my bets on "billionaire interracial sissyboy" over a conventional "biker billionaire" any day.

If your book is live it's really easy to see how effective your keywords are, just type in the exact keywords you used in various combos (I think any reasonable buyer will only type a maximum of 3-4 "actual" keywords like "billionaire biker bareback"). If your book doesn't show up then obviously buyers aren't going to find you either.

You might get lucky if your title is on point and your blurb is stuffed with the right keywords but you're leaving money on the table.

The other aspect about keywords is how competitive you and your book are in that specific niche. You will not outrank established authors with "billionaire", "biker", or "motorcycle club". Those people are probably living in mansions getting fed chocolates by naked Abercrombie models. But "futanari hentai sissyboy billionaire"? Well, maybe. All you have to do is see if there are any books with decent rankings with specific keyword combos and write a book.

I've written books with plots designed around certain keyword combinations that did great even though the interpretation took a lot of creative liberties without crossing any boundaries. You need to experiment more. Read W2LF's books for sure.