Everyone is different. Some people who are non binary but lean/present as feminine might use feminine words, but others I’ve seen use the masculine because they think modifying the grammar is complicated.
I personally use the gender-neutral modified language, like using elle in Spanish.
Not a native speaker of French, but while “on” can be used as singular “they,” it is much more often used to say “one,” as in: “One could eat a meal.” I think “iel” is better because it is distinctly a third person singular personal pronoun. Sure, it is not as well established, but it is in at least one big respected dictionary.