In the end none of it works. The best predictor for number of kids is still lack of money - the poorer one is, the more kids one has.
Of course the solution can not be to get poor in order to save civilisation
But there seems to be precious few workable solutions.
Personally, on a 50 plus year horizon, I am hoping/counting on revolutions in biomedicine and AI and technology. More concretely, in a combination of longevity progress and growing babies outside the body, cared for by robots. If one read Asimov's Solaria - well something like that. The thing is, it actually seems possible - in a 20 to 50 year time scale:
- cheap, caring, smart, ubiquitous robots - check - Tesla will deliver.
- longevity - there is a HUGE amount of ongoing research addressing the root causes of death, and there is huge progress in achieving significantly longer lifespans for mice. Serious trials for humans are ongoing. In principle, there is no fundamental reason we should die, if we identify properly all the reasons cells age, if we can return old cells to the younger state (Yamanaka factors), if we can eliminate senescent cells, etc - and there is truly impresive progress in these fields.
- growing babies outside of uterus - there is also significant progress towards creating artificial wombs, and I would be surprised if it is not realized within 50 years.
I am not saying this is a GOOD future - frankly, many aspects of it seem completely terrifying. I am saying though that it is a realistic solution to the depopulation catastrophe coming upon us. And actually a likely future.