For me, one of the ways I found out I was genderflux was that I knew I wasn't a man. So, "girl" or "woman" must fit me then, right? And it wasn't so much that I completely didn't identify as female, it's just the term didn't fully describe me. There was some extra stuff, experiences, and feelings that were being left out. It mainly came down to, "Well, I'm not a man, so I must be a woman." But, now that I've found this term, its sort of all-encompassing and more flexible than just "female."
If you feel better with people referring to you with they/them, then that's awesome! Try it out, get a feel for it, and see where things take you.
(P.S. There's also a term for both genderfluid and genderflux combined called "fluidflux," if you wanted to check that out!)