I disagree with what you are saying here on both counts. There are tons of sources. Some mention the conversion, and others just reference the accepted fact at the time that all Khazarians were Jews. These come from sources across all Abrahamic religions. The fact that these historical claims were known for hundreds (or in some cases over 1000) years but were apparently never contested is actually quite telling. If a historical author stated something as a known fact but it was not a known fact, we would expect him/her to have been corrected on this, if only by actual Khazarians. But there is no evidence I have seen that anyone ever had any problem associating the Jews with Khazarian converts until the 20th century.
I disagree with what you are saying here on both counts. There are tons of sources. Some mention the conversion, and others just reference the accepted fact at the time that all Khazarians were Jews. These come from sources across all Abrahamic religions. The fact that these historical claims were known for hundreds (or in some cases over 1000) years but were apparently never contested is actually quite telling. If a historical author stated something as a known fact but it was not a known fact, we would expect him/her to have been corrected on this, if only by actual Khazarians. But there is no evidence I have seen that anyone ever had any problem associating the Jews with Khazarian converts until the 20th century.