Overwhelmingly, across different societies, with different efforts to tweak variables, the result is that pregnancy and childbirth are risky and unpleasant enough that the average woman, given the choice, doesn't want to do it twice.

Fertility used to be higher because women used not to have that choice. At this point, if we want to grow or sustain populations, the only possibilities seem to be

1. Take that choice away from women. Not only would this be abhorrent, I'm not sure it's even possible without some sort of mass violence or horrific war.

2. Bribe women to have children, above and beyond the (economic) cost of having them. This seems difficult, and I genuinely don't know how high you'd have to go to get to replacement fertility. If you're not a woman, genuinely imagine how much you'd have to be paid to give birth to two babies you don't want. Then add the economic opportunity costs to that. Do we really have the resources to give that to half the population (because how do you know who wouldn't give birth without it)? Plus, a lot of men would be very mad.

3. Massive government investment into obstetrics to make pregnancy and childbirth dramatically easier on your body. This, to me, seems the most plausible, though there're obviously still major social barriers.

4. Develop sci-fi tech that removes or reduces the obligation for only women to bear children - either by inventing make pregnancy (halves the necessary average fertility, plus it's much easier to convince people who haven't done it before to have a baby) or artificial wombs. This is pretty far out, but I'm not aware of any actual hard limits on the possibilities. From my perspective, it's probably easier than stopping aging, which looks to have some genuine enthropic challenges.

Everyone (including me) is inclined to blame lower birthrates on their pet social cause (economic inequality, cost of housing, "The LGBT agenda", cars, cities, foreigners, the job market, social media, feminism, Marxism, conservatism, obesity, vaccines) but just as an example, the reduction US birthrates has largely been driven by a precipitous drop in teen pregnancy. As hormonal birth control and sex education has become more available, it's been easier and easier for women to prevent unwanted pregnancies without the cooperation or involvement of men, and birthrates have, predictably, dropped.

And I think it's probably going to be pretty hard to put that genie back in the bottle, unless you can get women to vote against their own right to vote and weather the inevitable storm caused by telling 50% of everyone they're not really people anymore and should just do what they're told. Women tend to be less violent and less physically imposing then men, but I don't think they're actually much less capable of causing destruction with, like, a petrol bomb, and I think we would probably find the line that overcomes that tendency pretty fast if we went down that path.

Your solutions are mostly to the birth issue, but i think there is an extra burden which is child rearing. The opportunity cost goes way beyond 9 months and even with both parents, raising more than one child is very demanding and the male may also be against further children. So women are not the only obstacle, males will also be.