I think they are all the right questions. There is the desire to have children and then there is the desire to have sex without having children. Condoms have a 12% failure rate in real world practice over the course of a year. I am sure they are better today than in the 90s, but I had my share of them breaking on me. It was common enough for me to cause some anxiety. Similarly, one of my children was conceived while using the 'pill' -- statistically 9% effective over the course of a year. Anyway, my wife and I both come from a long line of extremely fertile people, her mother was conceived 14 years after a tubal ligation while her parents were in their mid 40s. Probably TMI, sorry. Anyway, my kids I have had a lot of talk about birth control.
Bringing it back on to the general level, I am sure you are right that economic and social changes are making it less desirable to have large families. On a subsistence level farm a child is another hand, in a city a child is another person to feed, educate, etc.. The pressures keep going in the direction of fewer children*. Also part of the equation is also the spread of better birth control technology than was available a few decades ago.
*It might be that in a few decades only the wealthy will have children -- that's a dystopian thought, but I can see it happening.