I'd argue the mass death of German men and the prostituting of German women to the Allied armies that caused the issues. Kind of hard to birth a new generation when the best men are dead and most the survivors are mostly those guys who weren't fit for military service or older guys (sperm quality drops off with age) and trauma and various chemicals people were exposed to during the war likely did not help with epigenetics. Seems like the charts that note that he problems starting in the 1970s would actually fit that theory.
I have a theory too that Feminism is causing a huge part of these issue, because educated career women, the smartest, delaying having kids so only have 1-2 if they have any at all (most don't). Given the collapse of birthrates down to levels not seen since the hyperinflation of the early 1920s, it seems that is the key issue. Immigration may not be helping, but it is a lower order effect IMHO.
Then there is the issue of pollution as well and perhaps even radiation from all the electronics we're using. Not to mention everyone getting fatter, lazier, and less social. Overreliance of computers is not helping either.
Feminism certainly has an effect. It would undoubtedly be easier for more women could be career women while simultaneously having and raising lots of children, similar to, say, Amy Coney Barrett, who has seven children in total (five biological and two adopted) while simultaneously being a US Supreme Court Justice. Perhaps artificial wombs could help with this in the long(er)-run if they will be developed and commercialized up to the point that they will actually be cheap enough for ordinary people to afford.