Yeah, I'm not for gender abolition. I think gender is a meaningful concept.
I know people who go by they/them and refer to everyone around them to go by they/them as a kind of anarchist expression of enbyness. I think that's an interesting idea, but I made friends with binary trans people before I made friends with nonbinary trans people, and it needs to be something that others around you are OK with signing onto. In particular there is a strong tendency for haters to describe binary trans people as "they" as a way to delegitimize their chosen gender (this especially happens to trans women, but I've seen it happen to trans men, too). See also calling people "it" who did not indicate they wanted to be called that.
But what that says is: in some people's mind, being nonbinary is the delegitimizing of gender. It isn't a position in and of itself, not something you can affirmatively choose to become. It represents the removal of gender.
There are also binary trans people who transition not due to gender dysphoria, but due to what is called gender euphoria. How they are is fine, but they think it would just be STELLAR to be a girl/guy instead. Pain is not what legitimizes gender.
The answer to your question is that being nonbinary is actually ANOTHER umbrella. It includes gender anarchists, agender people, bigender/genderfluid people ... and me, who got so tired of being beat up for doing my gender wrong and feeling incapable of reforming that I completely gave up and made myself a new identity. One that better matched who I was and a person, and which I could affirmatively choose.
For me, that's literally a 0% ideological decision. I am happier identifying as genderqueer and I genuinely feel it is safer (because I wasn't interested in defending myself when I did my best to kowtow to the altar of Female, and now I no longer just take it when people hurt me, because the thing I have to protect, my self, now has a non-zero value to me). When I act the way that I used to before I forcibly changed all of my physical mannerisms, interests, and fashion sense in late high school for ~marriage~ and ~finding a masculine man~, I feel I am often-to-usually gendered correctly in public. Despite my long hair and lack of bodily modifications.
The last part of your question sounds like being gender nonconforming, which is considered part of the trans umbrella because it represents a chosen deviation from societal expectations of gender performance. In some ways being queer (i.e. gay or lesbian) is a kind of transness, except that it may be restricted to the domain of sex and romance. Transness radiates out from there to incorporate other deviations. And, being nonbinary describes deviations which are intentional in their expression of chaos, absence of gendered behavior, or fluidity, or neutrality.
Essentially, think about the things that a cisgender person is supposed to expect from their gender
as a category.
1. They are supposed to perform some kind of gender. Not performing gender is not an option.
2. The options are male and female. There are no other options. Not, for example, a spectrum.
3. They are expected to perform the gender that matches what the doctor said, not one they choose or construct.
4. That gender is supposed to be stable across time (nonfluid).
5. That gender is supposed to be stable in the moment (not pulling at any particular time from the toolboxes of other, legitimized genders).
And then more rules for trans people specifically . . . which universally assume binaryhood.
6. A good trans person aspires to pass.
7. A good trans person spends all money and social resources available in order to pass.
8. A good trans person recognizes that they are not what they have become, and should restrict themselves from normal activities that every other person of their gender gets to participate in as a matter of fact. Such as: using a public restroom. Or buying clothes. Or expecting that a public restroom in line with their gender be available.
9. A good trans person understands that nobody wants their child to be like that and restricts themselves from public spaces accordingly.
10. A good trans person understands that nobody wants to be around them, hire them, or house them.
11. A good trans person does not reproduce, either biologically or by acting as though it is acceptable to be trans, and *especially* not after transitioning.
12. A good trans person has a story that explains why they are trans. By necessity, that story makes it clear that the trans person knows transness is unacceptable. They are requesting clemency by telling this story which explains their transness; making it clear that they would not be trans if they had had any other choice.
Must I go on?
(nobody, everybody: these refer to the expectations and experiences of the "straight" world)
Why choose not to do any one of those first five things? Well ... lots of people have their reasons. Some are political, some are probably biological, some are based on experiences (stereotypically, trauma), some are based on about as much decision-making as one might take when grabbing a t-shirt out of the closet. I.e. simple preference.
Anyone can use any pronouns that they want. For any reason. They can wear skirts and suit jackets, cut their hair this or that way, or seek out hormones for as long or as little as they like.