First, there is no connection or relationship between the unleavened bread matzos) and the “what-is-it” substance (manna). These substances are not even physically alike at all, even though manna is sometimes referred to in Scripture as “bread.”
This is not to argue against what you are saying; I am not familiar with the symbolism used in the Seder meal. But if there was a desire to prepare a substance to symbolize manna, what would you use to represent this “what-is-it”?
Bottom line, as the coffee goes to work, manna was something real that God gave to the Israelites which could sustain their bodily functions, The matzo was a regular bread dough that was cooked before it had a chance to rise from yeast working in it. This was meant to symbolize the rush to get out of Egypt (the Israelites were to eat this last meal in Egypt with their sandals on, if you get my drift). They were also to have made enough of this matzo so that they had ration for seven days with them we they fled from Egypt. Hence the Passover matzo is eaten for seven days in the Remembrance Seder re-enactment. The matzo is not related to the manna. Jesus was sharing the matzo in the Passover Seder with zero reference to manna. This should give clues to a Bible student for understanding the exchanges in John 6.
The Passover meal, Seder, is simply a regular yearly memorial yearly observed (1) on the month and day that the Hebrews avoided the Death Angel and left Egypt under God's protection, and (2) used the same food that the Hebrews ate in their hurry-up departure meal. So the Seder is meant to literally replicate the original meal. The Jews have invented and included other symbolic features, like hiding the leaven, and an extra seat for Elijah (who came long after the Exodus), but these are embroideries of the real truth of delivery from oppression by crying out to God.
. . . what would you use to represent this what-is-it?
Manna has nothing to do with this, but was food supplied by God from Heaven, and I don't know why one would even want to try to duplicate or revive something that would turn to wormy garbage if it was kept overnight, and I don't know what the purpose would be. I wouldn't waste time on that, nor of inventing shoes and shirts that wouldn't wear out in forty years. I'd get kind of tired of all this, and want some new clothes, and onions, and garlic as well. Apparently manna was pretty bland and without nuch texture.