I want to see more sci-fi fantasy duos because it’s either just sci-fi or just fantasy, or fantasy romance. Romance is always the top subgenre and it gets tiring after a while. Relationships aren’t really pizazz-y anymore lol.
I do because it’s a big part of your story, both in plot and market. And I typically do. I’d say always, but I had a book’s genre shift when it wasn’t written the way I’d intended. I wanted to write a horror novel but it turned into a murder mystery.
Otherwise, when I have a genre in mind, it stays that way.
Honestly, no. Because truthfully, the only sub-genre you should be marketing is the main one that overpowers the rest. For example, the Hunger Games is a dystopian series with a sub-genre of romance. You can also say action as well, but technically, as a reader, you can see that it’s clearly filled with action so it kind of clumps together with the dystopian genre. Romance is its main subgenre because throughout the original trilogy, it semi-focused on the relationships between Katniss, Gale, and Peeta. After all, part of the reason why Katniss survived was through the sponsors who enjoyed her star-crossed lover romance with Peeta.
But I digress.
I think it actually helps the reader (or at least me lol) in figuring out what they want the most out of the book. Because sometimes, you want to just read a fantasy action novel, not a fantasy romance novel where the romance overpowers the fantasy.
No. I think that it made it harder to find books. 
I think there once was, but I don’t think it’s that apparent anymore just because, as you said, it sprinkles itself into multiple genres.
Action and thriller are their own genres because there is a difference, though they can be lumped in with other genres.
Action, for example, are usually contemporary stories that focus on a lot of high-chase action, guns, explosions, etc. Something like James Bond. And thrillers tend to be darker contemporaries that focus on suspense and tension, usually overlapping with crime, horror, and mystery.