There are agents who handle several genres. You could do your first submissions to those multi-faceted agents, making sure you pick one who specifically handles thrillers, since that's the one closest to completion.
Now, writing in several genre will not decrease your chance of making the first sale. You don't even have to tell the agent or publisher that your next work will be, not a thriller but a genre Romance.
However ... working two or more genre will affect your long-term sales. You will not be able to sign multi-book contracts, for instance. Your S.F. fans will have to wait and wait for your next S.F. book while you are off writing a Romance or Mystery or whatever. Those S.F. fans may forget you after two or three years.
So your agent may encourage you to stick to a single genre in which you are successful, at least until you build a reputation and a following. There are commercial reasons for sticking to one genre.
Many authors write in more than one genre. Within arm's reach is a J.D. Robb and a Marion Chesney.
Go for it