Finished tuning densities for other eco-adaptive models and then reviewed settings for agricultural as well. Here they are pictured in order, if the entire world was populated only using the models of Foraging, Nomadic Pastoralism, Cyclic Pastoralism, Settled Pastoralism, and Agriculture, respectively. The total world populations in such cases would be:
Foragers: 24,781,828
Nomadic Pastoralists: 97,351,871
Cyclic Pastoralists: 131,637,283
Settled Pastoralists: 107,898,619
Farmers: 72,337,495
Obviously these values look wrong (for the later types - forager & nomadic estimates are reasonably ok) - and this is because currently all the soils in the world are Regosols, which are perfectly "in the middle" so the EAMs that rely more on soil quality (particularly Settled Pastoralists and Farmers) are moderated a lot. Agriculture has a higher maximum, but it's also more sensitive to terrain restrictions than the other models. Testing with, say, turning the entire world into Chernozemic soils yields a Farmers-only population of nearly half a billion and also drastically boosts Settled Pastoralist outcomes. I will paint soils into the world eventually (probably in the near future actually so that further generation testing makes sense), and between that, eventual inclusion of rivers, and adding agricultural development levels, eventually the main centres of civilization will have drastically higher populations.
I also note that this world is generally going to have a higher carrying capacity than Earth, because it doesn't have the large swathes of largely unusable subarctic land that Earth does in the Canadian Shield and Russian Far East, and has substantial subtropical (Cfa/Cfb/Cwa) regions that are basically optimal for human habitation, especially forming a continuous arc along the north. It also doesn't have some of the global travel issues that Earth does (it's hard to get from between North America, Europe, and China until technology permits it) so high-power crops like rice and potatoes would be spread a lot more early, which will again eventually lead to higher populations.
In any case, moving on to figuring out how to integrate the current population estimates with the existing generation code. It'll be a bit finnicky because culture data is done per-channel (sub-tile, basically) but all population estimate code is based on averaged data for that tile, so I'll have to sort out which stuff can be calculated in advance and which stuff needs to check the culture's EAM.