I have to question the point of this thread. First the Church has already said that leavened bread is valid matter. It is the norm for some Eastern Churches both Orthodox and Catholic. Further, even the Roman Catholic Church can use leavened bread for Communion if a case of necessity arises (ie. you’re on a cruise ship in the middle of the ocean with a priest and its a Sunday and ran out of unleavened bread last week).
Questioning the type of bread used at the last supper only leads to the follow-up question of:
a. if its unleavened, then is leavened bread invalid?
Or
b. if its leavened, then is unleavened bread invalid?
Why create disunity where there is no problem today? The Church in her wisdom has already made a call on the matter. No need for us to speculate otherwise.