I see your perspective but I think that it is basically wrong. Your worldview misses a few key points:
1. The human growth explosion already happened. We're dealing with the end stage of it over the next 50-100 years. There will never be a world of "a hundred billion people on Earth" since it turns out people don't want to reproduce that much. We'll hit peak human this century which means that we actually do have a shot at giving all people a decent lifestyle if we develop our technology and have abundant energy. Exponential growth is just not required. The Ehrlichian doomsday was never going to happen.
2. The past was actually really bad, even for the wealthiest. Disease, starvation, and violence were actually rampant. Rewinding the development clock in rich areas, or pausing it in developing areas, would be equivalent to the largest crimes against humanity ever perpetrated.
I find your rejection of this basic fact of history by writing things like: "if native peoples lives (over hundreds of thousands of years, indeed) are such utter trash that they need to be rescued from it by technology" or "Incredibly tone deaf, frankly, when most past humans had none of those things and yet dealt with it, living actual full lives. We've basically forgotten what we are, what we came from, and believe we deserve Saturday morning cartoons." to be wildly ignorant. These people suffered, for millions of years, and still they suffer today. Alleviating this suffering is not optional. Frankly only someone who grew up in the utterly privilege western culture could ever think or write something like this.
3. Humans are not separate from nature. This one is counter-intuitive and hard to swallow but the truth is all human activity is natural, Lowes and Burger Kings are just a natural as forests. Nature is utterly impersonal. It has no morality or opinion. Sometimes one species gets a lot of adaptions that let it dominate and reshape the rest of nature into the way it sees fit. Then other species evolve to fill the new niches that are created. We make a lot of plastic now, but bacteria are starting to evolve to eat it, is plastic bad for nature? It's a nonsensical question. Plastic is nature.
It's fine to not like this and prefer some parts of nature over others, I certainly do, but no part of it is "better" or "worse" it just IS. Any morality you assign to it is purely your own opinion. You're right that keeping 8 billion humans fed and happy has a big impact on the rest of nature. But the only moral dimension that impact has is: will it cause more or less human suffering?
Sometimes the answer is yes, and we should stop. Sometimes is no and we have a moral duty to reconfigure nature to our liking. Most of the time the answer is: "it's not clear" and then people argue about it and sometimes pass laws.
But crucially it's all about humanity.
> And if you take my opinion for advocating any fascist-sounding, eugenics-sounding, kill-the-baddies-sounding recommendation, then you're reading something in I didn't write
But surely you must realize that the malthusian collapse that you anticipate will be characterized by authoritarian violence. Times of resource scarcity always are. So by embracing a defeatist philosophy you implicitly endorse it.