Nazi German demographic policy is going to be hugely complicated, as will the consequences.
The whole question of who will, and will not, be counted as "German", matters. Will the Dutch and Flemish be considered German, with everything that implies? What of the Scandinavians? Will Slavic children with an Aryan look taken from their families be included, and how will they be included? Will the naturalization and assimilation of adults of suitable backgrounds be possible? Without a clear answer as to who will be and who will not be included as German, any answer as to the long-run demographic future of Germany will be very difficult to come up with.
The many likely contradictions in German demographic policy will also complicate things significantly. The Nazis were committed to a program of traditional mores and very conservative gender roles, and also to a program of extensive rural colonization in the east. In theory, a substantially more rural and conservative Germany might be one with a younger and faster-growing population. Then again, Nazi Germany was also committed to a program of rapid economic growth and improved living standards, of the German nation to the high living standards of the Americans. Is it even possible to have a more rural and conservative Germany that will at the same time be richer and more modern? What will give? Will a country that already experienced liberalism in the pre-Nazi era actually be likely to embrace traditional cultural norms and demographic patterns? Is Nazi Germany going to be run by people capable of overseeing the sort of economic growth that will support a massive boost in fertility?
The only example we have in OTL of a country with coercive demographic policies, that of Romania from the mid-1960s up to the 1989 Revolution, suggests to me that Nazi pronatalism is not likely to result in much change. The imposition of Ceaucescu's policy in 1967 did
result in a baby boom, with fertility rates rising in one year from below-replacement levels to rates last seen in the interwar era. In subsequent years, however, Romanian fertility and birth rates continued to decline, by the early 1980s reaching the below-replacement levels of two decades earlier and then shooting up to levels slightly above replacement levels for the remainder of the 1980s. (Afterwards, fertility and birth rates collapsed. I think the legalization of abortion was one of the first things the post-Ceaucescu government did.) Romanians did have more children to start, but they adapted. Even without modern birth control and abortion, it's entirely possible for people to limit birth rates, through coitus interruptus and like measures--the example of pre-1940 France comes to mind. The Romanian government failed utterly to create an environment that made Romanians want to have many more children, and so, no matter how tightly the Romanian government tightened the screwed, birth rates failed to stay high indefinitely.
East Germany did embark on an interesting pronatalist policy from the 1960s on, but in many ways it was the opposite of the Romanian and the likely Nazi German. Instead of forcing East Germans to have more children than they wanted, East Germany's government tried to incentivize childbearing, linking larger families with higher standards of living (access to bigger apartments, say) and supporting non-traditional families (working mothers, for instance) with childcare and other like programs. A Nazi Germany that is devoted to teh causes of cultural conservatism is not going to echo the German Democratic Republic in any significant way.