Well, “My Liege,” “Your Majesty,” “Your Grace,” “Your Honor,” “Your Worship,” and “Your Highness” are all gender-neutral. I also like “Comrade” for addressing someone of the same rank and “Friend” for addressing someone of slightly higher or lower rank (i.e. “Friend Carson, it’s not what I can do for you, but what we can do for each other.”). I’m sure other writers will weigh in with suggestions of other real-world gender-neutral honorifics and/or titles.
You might look outside of English honorifics as well. Here’s the Wikipedia for Honorifics, which will link you to how honorifics work in different cultures and time periods, including in Chinese, Japanese, Tamil, Italian, Korean, Thai, Byzantine, Islam, Nahuatl, Jewish, and French.
You could also make up your own gender-neutral honorifics. Stephen King, Dan Simmons, and George R. R. Martin, among many other authors, have done this for their fantasy novels.
Also bear in mind that the trope She Is the King exists even in real life, and that a few historical figures have appropriated traditionally-gendered titles for their own devices over the centuries. (A few examples: Hatshepsut ruled in ancient Egypt as a Pharaoh, a masculine title. Elizabeth I ruled England as with the masculine title of Rex instead of the feminine Regina. Elagabalus, a ruler of Rome in the third century, preferred “Empress” over “Emperor.”)
Still, I think these honorifics are pretty gendered. Specifically, I think the women who used traditionally masculine honorifics were borrowing authority from them, because women were not usually recognized as worthy leaders—at least, not as worthy as men. Even your example titles are masculine, probably because they feel more powerful than feminine honorifics. To our Western ears, “Prince” feels like it has more intrinsic power behind it than “Princess,” its feminine counterpart, even though they’re technically the same rank. You could just as easily use “Princess” as a gender-neutral term, but I bet it feels like a weaker and more gendered choice than “Prince.”
To answer your question of whether conventionally masculine or feminine titles can be used as gender-neutral, my gut feeling is that it wouldn’t work as well as you’d like it to, but it’s really up to you.
You could try out using masculine honorifics for nonbinary characters and see how it feels. Maybe get some feedback from readers who identify as nonbinary. If you like how it goes, keep them! If not, use something else. Again, all of this is your call. It’s your story. Make up your own rules.
Thanks for your question!
-C