Nonfiction is usually easier to sell than fiction. Many busy people don’t have time or inclination to indulge in fiction, yet will read for information. There is also less competition, at least in some disciplines.
If you have a unique niche, such as a craft or hobby, or some area in which you’d be considered an expert, don’t hide your talent under the proverbial bushel. Having a blog, YouTube channel, podcast, or maybe even some books or ebooks out on that subject can help the cash flow as well as draw in new fiction fans.
For me, as you’ve probably already figured out if you’re a regular reader, it’s astrology. Technically, I’m retired, but my “day job” comprises being a professional astrologer, something about as antithetical from being an engineer at NASA as I could think of. Not only does it supplement my income, but I have found in some places and situations people are more interested in astrology than science fiction, giving me something to fall back on if sci-fi isn’t in their wheelhouse. How many people do you know who don’t know their zodiac sign?