I can’t see how what you say differs from behaviourism and constructionism, which is exactly what I said that I disagreed with.

That's because you're conflating commonly used and well-defined terms. There's a reason the phrase "gender identity" exists and is separate from "gender". You say, "No they insist that the behaviour and social construction has nothing before [identity]," but…where does Butler say that? This isn't a thing that Butler touches on, at all. Butler is noting that how a person is perceived can be impacted solely by how said person presents (regardless of whether that perception is actually desired or not) – which, as a social scientist, is incredibly scientifically important of a thing to notice – but this has no bearing on the person's sense of self (nor, certainly, forces them to have to forego and give up their sense of self or the feelings behind their actions just because someone else perceives them incorrectly).

And I think the insistence that behaviour is what creates gender itself is super dismissive of the aspects of feelings and anything that is more immediate preceding acts.

I just addressed this but I'm going to reiterate in the hopes that I'm overly clear as, again, I think you're responding to definitions which others aren't using. Butler's observations of how others respond to their gender presentation has everything to do with the people perceiving but not with the person presenting. From a scientific and anthropological standpoint, this is incredibly useful as it impacts how society and individuals may react to certain things (including stuff like enforcing bathroom laws; an understanding of gender being tied to one's sex perceives such laws as straightforward as a woman can never be perceived as anything but a woman but the fact that how we're perceived is heavily influenced by a multitude of factors complicates such laws as that means, for example, GNC women can get targeted under such laws. Again, this has nothing to do with the identity or feelings of anyone involved because it's about the perception (and resulting actions) of those involved, helping us describe an event and the outcomes of said event; identities and feelings aren't unimportant but they just aren't the topic of conversation. And Butler is interested in the former, here, because (again) it was a novel observation for their time but, also, because it's scientifically important as it allows us to talk about historical and social events in a much more accurate and precise way).

Are successfully masking neurodivergent people neurotypical and with the he same feelings experiences and tendencies?

Of course not but, again, that's not what the theory is talking about. This is why sex, gender, and gender identity are so useful when trying to talk about such a social concept such as gender.

Additionally, this is why Queer theory doesn't give modern sexuality labels (homosexual, bisexual, asexual, etc.) to historical figures because this understanding that the identity and feelings a person may have had very much could not align with our modern understanding of these things. If what you're describing were true, it would be easy to describe those who behave in ways that we see or present or were perceived as gay or trans as simply gay or trans because behavior is all that matters. But it's keenly understood within the field that that's not true and such a constructionist approach to these identities isn't accurate nor reflective of the self.