This just started bugging me and I got no idea what the answer even remotely is, its likely been answered already but I'll shoot a shot in the dark regardless:

What is the term used when the sub is non binary? Is it something more descriptive like "doll" or do we go hail Mary with "them?" This shouldn't be as interesting of a question to me as it is right now but I just gotta know.

i think this is actually a question that They have been trying to unravel for some time now, to be honest

Indeed. Many good scientists, hypnotists, and hypnotized scientists have been working tirelessly around the clock to solve such a question. Last I heard they hope to come to a conclusion by the 2027 4th quarter.

Preliminarily, the answer they come to is this: 'sub is a state of mind. An NB sub is still a sub. After all, any orientation is allowed to kneel and let the bliss wash over them~'

Easy.

Whatever term the sub likes and is comfortable with.

With enbies you can use "Toy", "Doll", "Slave" or even just "Enby". Or maybe they prefer some stronger language. But really the answer is just asking what they prefer before any sort of session.

Good answer. I would use "submissive little pet" as a gender neutral term.