That's not nihilism, nihilism is a specific philosophy that is often misunderstood. Here's the three main branches, you could say, of nihilist philosophy:

Nihilism: Life has no meaning, so you must find your own.

Existentialism: Life has no meaning, only your actions/choices do.

Absurdism: Life has no meaning, let's have some fun while we're here.

I don't see this as nihilism. Additionally, I don't see why adoption needs to be contingent on life becoming permanently better for all humans. If you want kids, and are able to provide an adopted child with a better life, I see that as a win-win. Under antinatalism, if you do not have any children already, the choice of adoption just reduces to the childfree case (e.g. do you find them annoying?). Therefore, your thought seems pretty much to line up with antinatalism.

Correction: humanity doesn’t have consistent track record of making things worse for each other, it’s all on civilaztions that have ever adopted farming. There are still some fragments of communities living as hunter gatherers that base their life together on cooperation and caring.

Would you take an hour of the most excruciating pain for an hour of the most intense pleasure?

Our evolutionary ancestors needed to experience pain more intensely because it prevented them from dying. What does pleasure provide, when it merely acts as a tool for reinforcement?

You can try to refute my description of the pleasure-pain biology ^ , but once you accept it, I do not see how you could say "Says who?".

^ If you are refuting that, see this text explaining it biologically: https://www.researchgate.net/publication/5498867_A_common_neurobiology_for_pain_and_pleasure

Though there also seems to be a certain flavor of antinatalism which is something like "our universal shared conditions are going to get so bad that everyone will suffer" due to global warming, lare-stage capitalism, whatever. That one I can understand.

The flavor of antinatalism I presented above is not different from this. The two are not mutually exclusive. In my previous comment, I was more concerned with showing how the default state of life is negative. All these factors just add to the severity of how bad life is (and can be).

But if I start asking people on the street if, based on their subjective lived experience, they agree with that statement, I can guarantee that not everyone would.

People tend to remember pleasant experiences more accurately than unpleasant ones. This phenomenon is known as the Pollyanna principle. When you ask strangers about their lives, they are likely to focus on positive moments and downplay suffering. Yet, if you probe them, you might see that the reality of human existence includes chronic stress, job dissatisfaction, loneliness, loss, bereavement, discrimination, injustice, regret, disease, and ultimately death. These hardships are not uniformly distributed across a lifetime; they are usually amplified towards the end. For example, illness becomes more acute in old age, and many old people experience helplessness, loneliness, and memory loss, despite having had children.

And that's the rub for me: I see antinatalists taking this position as if "life is suffering" is subjectively true for all humans, and I don't think that's true, or at least not to an extent that leads to the conclusion that no one should reproduce in order to save the next generation from the suffering.

It is about preventing needless suffering. So, if suffering constitutes an overwhelming part of life, one should not procreate in order to avoid it. Sure, we may have our coping mechanisms to get around the suffering, but why learn and deploy them when it can be avoided altogether? It is not for the benefit of the child that they are being brought into existence.

So it seems to me that one driver of antinatalism is people with depression incorrectly assuming that the next generation will experience life the same way they do. All it takes to prove that assumption wrong is a single person who disagrees that their subjective lived experience is one of constant suffering.

I can similarly argue that people without depression assume that the next generation will do well too. You have already mentioned global warming and late‑stage capitalism. Given threats like these, I find it unreasonable to presume that future lives will be unproblematic. Reflecting honestly on ourselves and our circumstances reveals that we should not expose future generations to life and the world. You do not necessarily need to be depressed to recognise that things are bad.

Also, I am not particularly arguing for "constant suffering". My claim is that it is unethical to subject a sentient being to suffering when that suffering could be avoided.

The observation is fair: a population-level argument does not follow from one person's mood. The diagnosis is a different question from the truth of the argument. Whether someone arrived at the asymmetry or the consent argument while depressed changes nothing about whether its premises are valid; a depressed evaluator and a cheerful one assess the same argument by the same standard. The principle that emotionally-caused beliefs are suspect also boomerangs: life-affirmation carries its own documented etiology, an optimism bias that inflates the estimate of life's worth and is defended on adaptiveness rather than accuracy. Severe depression can under-value a life, but the only systematic skew on record runs upward. The worry about mood is legitimate in one form: naming a load-bearing premise and showing it is despair rather than reason.