I say that it’s actually impossible to answer, except in the most extreme cases.

It's certainly impossible to answer in a single broad stroke for everyone, but that doesn't mean it's not useful or relevant to think about.

That is because the question was about rights, not consent.

If that's so, then we're talking past one another. My point is that in my ethical framework, having a child is wrong. They are incapable of consenting as you point out, which is part of why I view it as wrong.

Existence is a presupposition to consent.

Why? The child surely exists before, during, and after its birth. Can't it be that the unborn human is incapable of consent rather than creating a paradox? I understand the chicken-and-egg problem you are describing, but I think it's incomplete. As a fully functioning human being now, if I look back at my birth, did I consent? Did I exist yet? I think I can say simply, "No, I did not consent to being born." Whether you ascribe a negative, positive, or neutral value to this is up to you. In my opinion, it's immoral.

In what way is that different to negotiation?

It isn't. I don't think we disagree on this

But birth is a natural result of conception.

Yes, however isn't this logic used to argue against abortion? I'd argue that a person becomes a full "person" at birth, which is perhaps arbitrary, but we have to define that point somewhere. Regardless of when we say a person "exists", they still cannot consent regardless.

That all said, is anti-natalism completely correct for everyone? I don't know. I'm sure our species going extinct would create lots of suffering for the dwindling population. Maybe on average, humans do not regret their existence. Does that mean it's moral to make more conscious beings who are capable of feeling that regret?