“but they always boast how they were always at least traditional and kept to marrying only Yidden. ”
That is true, you will rarely find a Atheist Sephardi. I assume, Ashknazim started going down the hill with the Haskalah, which originated along the secular non-jew as well, and advanced as the goyish society become more open and advanced as well. There is a reason why it succeeded in Germany.
Whereas, in Sephardi regions, the non-jew was much more reilgious as well.
You should read the Meshech Chachma in Parshas Bechukosei how he explains the issue of anti-Semitism.