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.
In French, the plural they is the masculine ils for mixed gender groups, and only the feminine elles for groups that are exclusively feminine. On is a singular gender neutral form of they often used in situations with unknown gender.
I would use on for singular and ils for plural when talking about a non-binary person because that seems the most reasonable application of the language as I understand it.
However as a non-native speaker I would defer to whatever native speakers do. Can’t say I know what the established best practice is.
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?