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.

carrots won't work for fertility because there's no gibs or bonus or benefit worth obliterating your life forever for. here's 5% off your taxes to never have fun again and spend the rest of your life as a sleep-deprived miserable slowly tortured caretaker.

you gotta just make it mandatory, people in cushy western countries won't ever freely choose to lower their quality of life by having kids. thirdies have tons of kids because they already live in misery and there's not much difference

edit: also eugenics would unironically help. you can't be like "we want you to have kids, we need them to keep society afloat!" but also be like "also btw if you get unlucky and have a tard baby we'll force you to stick with it for life, taking both parents out of contributing to society in order to keep alive a screaming violent blob that will never contribute either". that's just dumb.