Where does it come from the minhag to break a plate by a tanoim?
As well is there anyone who considers the seuda eaten by a tanoim now a days to be a “seudas mitzvah”?
Among Ashkenaz communities, there is a common custom to break a plate at the time of the tenaim.
Traditionally, the china plate is jointly dropped by the respective mothers of bride and groom.
The reason for breaking the plate is to demonstrate the strength of commitment to the forthcoming wedding, which is as final as the breaking of a plate.
Alternatively, the breaking of the plate expresses sorrow over the Destruction of the Temple.
A third possible allusio it to the coming “break” between the parents and their children, who will leave their homes to live with their spouse.
These are different ideas I’ve seen, though I don’t recall the sources.
The tena’im meal is not an actual se’udas mitzvah, and a little like Chanukah, it is customary that the groom deliver some Torah thoughts at the event, thus granting the gathering the status of a seudas mitzvah.