On the topic of comparing French and German fertility- so far as I can tell, Karlin isn’t making any attempt to control for Muslim fertility as a component of the French total? There are probably some ultra-conservative French catholics bumping up the total, but insular French-Muslim communities by themselves could explain the French case in roughly the same way as the Amish.
Also, FWIW, the lower rates of Amish defection over time might not necessarily reflect genetic changes (although that’s also possible.) The larger external secular society might just be getting more chaotic and repulsive over time.
I don’t think Hanson has any terribly useful suggestions for solving the secular fertility crisis, to be frank. The founding myth of our present-day secular elites is of course the holy crusade against the Nazis, which means that any serious national discussion about raising birthrates (let alone ensuring eugenic fertility) will get crushed by Lebensborn comparisons and other thought-terminating clichés.
Food and Energy Security should be rated much higher, along with self-sufficiency in general, since GPI and HDI could go to hell in a handbasket depending on what other countries get wiped out or rendered inaccessible to trade during a world war in general or nuclear war specifically. A lot of countries’ food output is dependent on fertiliser imports from places like Morocco and China.