I personally don't think journalists have been voicing these concerns in anything like a manner proportionate to the risk of megadeath (compared with something like, say, climate change, which has become a perennial font of public hysteria even though its impact on quality of life will likely be minimal for most areas of the globe.)
Climate change's effects on desertification and rising sea levels could plausibly either directly or indirectly kill tens of millions over the coming century- which isn't exactly good news, but... given our global population size, tens of millions could statistically die from various causes over the course of a century and the average person will probably never notice, especially when most of those deaths will occur in ecologically marginal areas where life was never easy to begin with.
If Zeihan's analysis is correct, then hundreds of millions could plausibly perish from war and famine just over the coming decade. If so, "keep global trade in fertiliser inputs cheap and safe" would have to be considered a project of overwhelming importance for rational altruists.
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.
An analyst I follow called Peter Zeihan managed to predict the Ukraine war and the impact it would have on global energy, food and fertiliser markets. He's now predicting global famine and civil unrest considerably worse than the arab spring before the end of the year, potentially including the collapse of China.
If this is true, there's a wide range of projects that need to be considered by way of making agriculture less fertiliser-dependent, and most projects aimed at, e.g, saving lives from tropical disease are small fry by comparison.
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.
I would strongly recommend taking a look at Peter Zeihan's work- he forecasted, among other things, the invasion of Ukraine back in 2015 based on an array of geographic and demographic factors, and expects many similar calamities in the coming years. (TLDR- Russia's top-heaving ageing demography and insecure geostrategic perimeter is prompting it to expand to reach geographical choke points like the Besarabian Gap before it runs out of young men to draft.)
There's an interview on the topic here that you might take a look at. Of particular interest and concern are his predictions of widespread famine due to disruptions of fertiliser inputs needed for agriculture in many parts of the world (due to natural gas and energy prices spiking, disruption of potash exports from Russia and Belarus, and phosphate exports from China recently being banned.) That's effectively an existential risk to civilisation as we would recognise it with or without nuclear weapons being involved.
I am baffled as to what 'net positive impact' extinction rebellion is supposed to have made, given they tarnish the reputation of the environmental movement as a whole with vastly exaggerated projections of doom, actively block pursuit of non-renewable energy solutions like natural gas, carbon capture and nuclear, and promote an anti-natalist hysteria which is only going to make other economic and social problems worse over the coming decades.