> populations in actual hypercompetitive hellholes, where they're not just competing for a job/status/basic financial solvency, but literally for their food security, physical safety, etc., are those which are having more kids
That's the problem we should solve. We don't want just "more children". We want "more children in the environments where there are resources available for their proper development". The "more children" in places like you describe problem is currently solved by more polytical instability, more war, more disease etc..
> "make raising kids cheaper/easier" and "make life easier" are not interventions proven to boost birth rates
Nothing's proven until you run an experiment to f prove it! You're the perfect example of "thinking prfoundly, but in the wrong direction" - under the whole flawed paradigm of "social science" you take the problems to be solved as "implacable natural tendencies" and from this you build flawed arguments against why the actual problems to be solved "can't be solved".
> The pretty-much-universal trend appears to be that once people become more educated and comfortable, they stop having kids above repopulation rate.
That's the freakin problem you need to solve, not a "trend" to placidly observe. We need to run experiments on multiple ways to alter/reverse this human behavior that's not natural but a product of the nasty society we've build for ourselves. OK, it was a price for a faster evolution towards post-industrial stage, but now we can tweak it and adjust the externalities.
We might want to start with the fact that people are rarely "educated and comfortable". Education often makes people slaves of social-loops where they need to work harder to keep the higher status they've got used to and so on. Most higher educated people are more stressed and less happy than lower education people. We need to give people stuff like "job tenures" etc. to create stability - the lower class people actually have this stability by virtue of being "rock bottom", eg. "it's hard to fall any lower down the social ladder, so at least you can lay back and feel good and comfy about it, with whatever rationalizations you can concoct, then start having some kids to get a feel of meaning in life, yey!".
We need to think active social engineering not passive social-"science". We've sold ourselved a bunch of feel good stories about "how things are" in our "society", instead of realizing that society is nothing but a mechanism with thousands of levers we can start tweaking until we get better outcomes...