In hindsight I should've probably not make 1b (1800) but rather the 1.65b of 1900 the goal, just because from a time frame and as a vision it is so much easier

In hindsight I should've probably not make 1b (1800) but rather the 1.65b of 1900 the goal, just because from a time frame and as a vision it is so much easier to communicate: around the turn of the century we already had electric streetcars, electric underground, electric lighting, motorcars, elevators, entire countries with rail networks and so on. I do not see why a slow-ish-ly shrinking population should entail tossing all of our advances since then—in medicine, computers, telecommunication, chemistry, virtually all fields, not least agriculture (although the way we do it now is utterly unsustainable). Rather, I believe that in a situation where you have an extremely high birthrate and a doubling of the population in a matter of just 15 or 30 years you end up with half of population being rather young and lacking experience; you will also, as an economy, have a hard time to build enough schools and housing. People will and already do live, in many places, in cities that are almost entirely without any built history to speak of. This is what erodes local traditions, customs, awareness; I see this and not the OP's notion of 'shrinking societies cause cultural attrition and global homogenization' (paraphrased from memory) as a major cultural problem. Population growth, not shrinkage, is a great contributor to cultural loss.