I have a small side gig as a freelance writer. It might be called "sub-barista" FIRE, as it's not enough for even the slimmest FIRE budget, but it's a nice supplement. I could scale it up somewhat, but that would mean taking clients I don't enjoy. The primary company I work for is openly asking for people who want to take more clients, but I've found that I enjoy some of the clients and others feel like slog, and I haven't yet figured out how to determine which one will be before I take it on. They frown on giving up clients, so for now I'm happy to stick with what I have.
It's 100% on my own time, and I can even write ahead 2-3 months if I want. (They do ask that i don't submit that far in advance, but that just means needing internet access for about 10 minutes during a window of about 10 days around the deadline.) So it really couldn't be any more flexible.
There are a couple other things I do, but they aren't [yet, at least] reliable or predictable.
It really is the ultimate FIRE part-time gig. I got it because writing is sort of My Thing, and because I happened to be fortunate enough to have a deer friend in an editorial position. She offered me my first published article, which lead to a couple more, and with that, I then had enough of a portfolio that all of the writing gigs that require one (which seems to be nearly all) were then open to me. So getting in was the tough part, then I fairly quickly got the steady gig I mentioned above.
So the take aways? I guess those would be
1) work all your connections. Let everyone you know know that you are looking for something. If you have specifics, let them know that, but I'd say it would be best to let them know to keep you in mind in case there is anything that comes up. Maybe it's well outside where you image you might end up being, so be open to anything. Tell your people to be on the look out for anything, and to feel free to pass your name along to anyone else who might come across anything.
2) Be open to anything. I said that above, but I'll be more specific since it can mean a couple different things. First, you might consider taking less desirable things if you have a sense they might lead you to something better. Consider them a stepping stone. Some people might object to this--it's FIRE and the whole point is freedom from unpleasant work. But if it gets you closer to pleasant work, it might be worth it temporarily. And second, it means being open to all sorts of things, rather than setting your mind on what you think you should be doing or are most likely to find. Maybe you have writing skills and assume that's the mostly likely avenue for you, for example. But someone out there is looking for a juggling chess player, and you happen to juggle and are a 1900 rated chess player. If you were only looking for writing gigs, you'd never have found that. So it's fine to have a focus, but don't get too focused.