The Republic of Venice never had an actual formal constitution, much less a written constitution. Neither did it have any kind of foundational event.
In the late Venetian republic, when the doge died, they held the funeral for a statue.
“Doge” from the Lessico Veneto (1851) by Fabio Mutinelli, translated by René Seindal.
The constitution of the Republic of Venice never really existed. The Venetian nobility just made it up as they went.
The Venetian state was always an ad hoc construct, and institutions came and went at the convenience of the ruling elite.
“Consiglio dei Dieci” from the L’Archivio di Stato di Venezia: indice generale, storico, descrittivo ed analitico (1937) by Andrea da Mosto
“Dose” from the Dizionario del Dialetto Veneziano by Giuseppe Boerio.
The doge was the elected head of the Venetian Republic. He was elected for life by an arcane process that also included drawing lots.