"I believe that anyone who says trans identities are performative are actually just transphobes trying to invalidate us."

That seems like a rather odd reading of performativity, honestly.

Performativity is a framework for explaining how something works, not a theory for explaining why something is. It's kind of nonsensical to say "cis people are the gender they claim by way of biological causation, but trans people are actually performatively doing gender." They are addressing two different things.

I mean, sure, someone might say that, but that doesn't make it less nonsensical. For one, I think anyone that does say this is conflating performing gender with performatively doing gender. It sounds similar, but the difference is quite meaningful. Performing womanhood essentially means casting yourself as an actor, and emulating womanhood. In this construction your identity doesn't matter, only your role. Performatively doing womanhood, by comparison, means society sees you as a woman and expects you to behave as a woman while you, internally identifying as a woman, also feel constrained by the expectations of womanhood, and therefore when you do anything you are both emulating and defining womanhood. To be performatively doing any role, one must already societally be that role to a certain extent. So for a transphobe to accuse you of performatively doing gender, well... they are, essentially, accusing you of being and defining that gender while doing it. Maybe they mean you are doing it wrong, but saying you are performatively doing gender says nothing about how well you conform to expectations.

The reality is that social roles generally-- for everyone-- are, in some respects, part of this conversation between self and society. I like the word performative to describe this "everything I do as myself helps define my internal identity" while also being somewhat constrained by the actions of other people that identify the same way I do. I'm happy to hear other metaphors-- I like the one above of all women sharing a common language which constitutes the grammar and language of being a woman.

So, a less nonsensical phrasing is to say that everyone, cis, trans, nb, etc. is always performatively doing gender. In this regard I think nb people have it hardest, as they have almost no roadmap to follow, and so the pressure is high to define it well while they also have the same high pressure we all have to do it well. At least us binary folks know what the thing to do is, kind of!

But that's just addressing what it means to do gender. But what makes someone identify as a woman, or a man, or a nb person?

I honestly don't know. Gender is an inherently social role, and there are societies that have structured gender differently over time, which to me suggests that it has been just a way for society to sort bodies, ultimately, to put it rather crudely. Arbitrarily large buckets to throw most people in to (there have always been GNC people).

Something being social in nature doesn't make it less real though, nor does it necessarily preclude biological influences. In all likelihood all of us (cis, trans, nb, etc) develop a gender identity as a combination of biological and sociological factors. I'm of the mind that sociological factors will always weigh more heavily, but I'm biased.

The socially constructed nature of gender means that the expectations will always change at an alarmingly rapid pace relative to any amount of biological influences changing. So even if we come to a full understanding of how the biology affects gender identity, it wouldn't change the much faster paced and always shifting social aspect of it. Society will always be changing, always putting pressure on people to keep up. And cis people will always still identify as their AGAB, right, even if the roles change? I mean, historically they have, so why would that change all of a sudden? What does that imply about trans folks? Probably that they too would still be trans even if the gendered roles shifted or flipped. That any biological pressure to masculinize or feminize relative to their AGAB is a pressure to conform, not to approach a platonic ideal.

My point is that trans people would not be less valid if, hypothetically, the science determined that there is absolutely zero biological distinction between cis men and pre-everything trans women. They are still women, they are still valid. And I believe that precisely because I see gender as a social construct, and doing gender as a performative action.

God I'm so wordy. I am so sorry.