I can't speak to places outside of the Western Anglosphere, because I simply don't have any information about them—but within it, it's abundantly clear (and there have been some recent studies bearing this out) that a major cause of reduced birth rates is lack of economic opportunity among the non-capital-owning classes.

I 100% guarantee you that if we implemented a full UBI today—one that would pay something close to the median individual income, and even if it were only for adults (and thus you got no "bonus" for having extra kids)—once the initial chaos settled down, you'd see those birth rates go up quite a bit. So many people are waiting to have kids until they're financially stable enough...and then they never become so.

I know that one of the big reasons my wife and I didn't have kids in our 20s was because we were concerned about our financial stability (and frankly, we were better off than most of the people that age today—by quite a ways, since we were homeowners).

Having a Scandinavian perspective on this, where birthrates also have fallen a lot recently, I don't think financial stability is the only factor. In my experience ppl here don't wait with having kids so long due to lack of money. With the social safety net we have here, free healthcare and education, paid parental leave etc, you are fine as long as you have a decent job. The reason that ppl wait until they are 35+ to get kids is rather that they want to do other shit first and just stay "young" longer and don't want the responsibility. Having kids is just not that important for ppl so they wait until they are kind of bored of other things and then they are often suddenly too old, or fail to find a good partner, or just don't want the responsibility.