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.
As french learner, I’m curious. How do you go about making adjectives neutral?