I think some sort of large scale geoengineering to achieve global carbon neutrality, maybe a genetically modified algae or cyanobacteria that would sequester a lot more CO2 than any current plant or bacteria.
Some people won't be able to do this due to the soil conditions and engineering resources. Those that can, should. It also makes for more usable land for a garden or crops and keeps your valuables out of sight.
Engineered correctly, I can weather any rain or fire storm. The real challenge will be building my underground garden. It has been done, just not easy at all. There are many other challenges that are not cheap to solve.
I know you wanted to solve climate change and this method would require mass participation. I just know humans well enough to know that anything bigger will take decades to implement. I can do this tiny step now.
That said, the place I am looking at is about 2000' feet above sea level, but may be subjected to snow and ice.
I am not saying you are wrong. Population reduction would definitely help but I don't see a simple solution like that working. It is much more drastic than a 1 child policy.
Assuming you have ways to enforce a global 1 child policy, it would only divide the population by 2 every generation, but it seems more humane. (But won't really help growth or financial prospects)
Anyway at some point, we will get the population reduction, voluntarily or not.