Some minor feedback on your rhetorical strategy: I think that you should reconsider your introduction. Accepting that kind of Stever Pinkeresque mainstream optimism doesn't set up your argument very well, which is more thoughtful and much more willing to engage with subversive and controversial ideas than I thought it'd be from the intro. I especially didn't like the law of diminishing returns bit: It's not obvious at all why one should be able to apply a concept from classical economics about productivity to the human emotional experience. Those are pretty different things, and if you want to make that case by analogy, you're going to have to set it up.
If it were me, I'd reframe the intro to be about how unquestioning optimism and a belief in a poorly defined "progress" is our society's common narrative. Instead of appealing to diminishing returns to transition to your point about antinatalism, you could instead cite modern scholars like Jason Hickel who are (I think) very convincingly rebutting that narrative. That tees you up a little better to take that big step.
That's really the main thing that jumped out to me. The only other thing that I'd say from my first pass is that you talk a lot about depression and the human tendency towards it. If I were reading this very closely, I would examine your sourcing on those points, and I would be extremely skeptical if your sources don't include anthropologists, many of whom I suspect (though I don't know) would take issue with that. It is true that it seems that we're becoming more depressed, but going from there to arguing that it's innate to humanity is, in my opinion, a big leap, even if you do cite some evolutionary reasons as to why it might be. It could that this line of thinking reifies hegemonic social conditions more than it says anything profound about humans.
Hope that helps!
I do really like the bait and switch intro strategy. I actually do that all the time in my own writing. I think that you could do that even harder using Hickel's work as a source. Something like "Global poverty is down. Famines are going away. Things are going amazing. Except they aren't," then on to your stance about antinatalism. That might even open up an interesting epistemological argument to pivot: We ought to focus on what we know, not on these fragile measurements and stories, and what we do know is that people are born and will feel pain and pleasure.
Haha oh boy do I feel this one. I try so hard to keep my own essays under 2000 words, which is where I notice that people tend to read them a whole lot less. It's really hard! I have no advice, since I'm terrible at this.
I actually think that's totally fine, especially for an undergrad, but also in general. I also think about this in my own writing. I love a big new idea and feel pressure to come up with more, but sometimes that's not what a topic needs. Sometimes things bear repeating with a new perspective, or a tweak, or a mix of other ideas. Sometimes, you just need to bring old ideas to a new audience, or repackage two relevant ideas from two places together, etc. All these things are worthy and necessary endeavors in human knowledge (re)production.