Good essay. Have you read The Left Hand of Darkness by Ursula K. Le Guin? That’s the book I took my last name from when I transitioned. All natives* of the planet Gethen are genderless, developing typical male or female physical characteristics for only a few days each month for sexual purposes. While there are no non-binary humans in that book either, it was published in 1969 and was pretty radical for its time.
* With the exception of a few who are permanently male or female, and face much the same discrimination as intersex humans do in real life.