> As of 2015, a total of almost 581,000 people have become legally Japanese since the fall of the shogunate and the rise of constitutional "westernized" Japan.
This seems consistent with other sources I found while doing a quick search.
This suggests less than 1% of the number of citizens of japan was not born a citizen.
Of course, some of those people will have children and those children's ethnicity could be not Japanese ethnically (or partially) but still the number of citizens of Japan that are not ethnically Japanese is quite small, and the number of citizens of Japan that are not ethnically east asian is much smaller still.
Also a lot of the Koreans in Japan migrated before 1940, peak in 1930. So the Koreans taking up citizenship are often third generation living in Japan, after beeing culturally fully integrated. So according to your numbers it would be fair to put the 'real foreigners' who took citizenship in the 0,1 to 0,3% range.
