First, the 10 other convictions whether misdemeanors or not, will likely be a serious problem. The fact that a particular crime is not a problem does not mean than 10 of them also are not, particularly in a very orderly society.

Second, expungement may be helpful in a US employment context, but it really doesn't mean anything even in the context of Global Entry or watch lists for US CBP purposes. It certainly means absolutely nothing overseas.

Rather than winding up caught in a lie and having a very unpleasant experience, before you drop a couple of thousand on a ticket, spend a couple of hundred consulting a lawyer who deals with Japanese immigration issues? Be honest with the lawyer. Don't bother characterizing crimes as "minor".