First of all the level and efficacy of health care in Aetas appears to be absolutely exceptional. Ignoring one current story thread healthcare is free, magical, fast and extremely reliable. I imagine their infant mortality rate isn't too far off from our own modern day rates. Also remember that men are highly incentivized to take multiple wives. If a guy has 5 wives and each of them has 2 children then that's 10 children being born per family. If at least one of them is a boy and that boy grows up to take at least 5 wives then the population would still grow. Can only do that for so many generations before the male population drops too low to both sustain and avoid inbreeding but it should hold over long enough to hopefully cure the curse.
That's nearly spot-on.
Like you said, the heathcare level in Aetas is exceptional, far far better than IRL. Cancer? Cured with a spell. Every single children diseases? Cured with a spell. Etc... There are some exceptions, like the withering, where you need more, but those are rare, and mostly magical in nature. The only medical procedures that you have to pay for are those that are cosmetic. And while no one HAS to pay, it's morally wrong to not give to the church of Marduk if you can afford it.
The only thing you are wrong about is that the infant mortality is lower than IRL. Marduk's clercis are far batter than modern medecine even dream to be, and the priestesses of Ishtar are all very well trained midwives, and collaborate with Marduk's followers so that deliveries go without an itch. And collaborated with the mages to study every facet of pregnancy and child-rearing to be as efficient as possible ever since the curse was discovered.
It's not well explained yet, but Aetas, and the world in general, lost a lot of improvments over the years. For example, skyships were common in the past. But they did not lose their magic. At least when it comes to arcane and divine spells. There was some knowledge lost when the Dwarven Empire fell, since they were the best enchanters in the world. So, while they are mostly medieval, a few centuries ago they were more at the renaissance level of technology, with magic on top of that. And they really, really, put the focus on beating the curse any ways they can. When you have magic at your disposal, a lot can be done. The demons actually underestimated the plane the world the story happens in, most others they conquered fell faster. But, there is only so much that can be done until the curse is broken, and the demons do use others means to reduce their numbers, like spycraft or supporting hostile groups.
As for the two children rules, nobody force you to have two children, but you can't access contaceptive before it, and they are controlled by the church of Ishtar. There is a black market, but the penalties are very harsh, death or collar.
And the Curse did not always reduce the ratio boys/girls to 1 in 10 right away. It was a progressive thing, and it continues to get lower.
.... Only a sex maniac would do that... Not a warrior on a mission
