That’s up to Germany, right? If racial purity is a requirement for being German then it will be harder to integrate immigrants. I’d suggest just dropping the requirement. The UK has much less of a problem with the concept of ethnically diverse British people (though it still has a long way to go).

Anyone who thinks that it takes ‘magic’ to turn someone who was born in Germany or Japan into a German or Japanese person is implicitly subscribing to some bogus theory of racially pure nations. (See e.g. your sibling comment, by someone who’s persistently been repeating far right talking points over the past few months.)