Hungary-like fertility policies flatly don't work.Hungary has dedicated major resources to this - no taxes for 3+ kids, debt forgiveness, major subsidies for homes. It's actually equivalent to ~5% of their GDP. The US military is 3.2%, so big spending. Nothing happened.

While I don't think economy is that important of a factor of fertility rate, poor thirdies actually have an economic incentive to breed.

Just one kid makes it, either on the continent or in US/Europe, and they're richer than they'd be childless, since it's so common and socially pressured to send money back home.