I don't see anyone having mentioned this, but wasn't a HUGE issue for regional parts of Italy was that the regional languages had little prestige (especially post Renaissance) compared to the Italian language itself, based on the Florentine dialect? Since Italy had to some extent a unified language amongst the upper classes, that means Venetian nationalism will be more difficult to develop. And Italy as a unified state was already imagined--Machiavelli certainly had imagined it. You need Venice to make its own identity. That will be a major cultural struggle (maybe comparable to the cultural issue in Tsarist Russia which you can still see traces of today, the whole "Is Russia a European state or something unique?") throughout the ages. For the "Venetianists", they can and should emphasise the maritime traditions of the republic (unlike the land-based city states and else) and the fact that Venice is not
just Italian, but has Illyrian ancestry or something. Possibly emphasising a Slavic element could work too (if Venice can assimilate the Slavic populations into being something like "Slavic Venetians" and not Croats or Slovenes or whatever, that would be very good), as well as a Greek element of Venice. Thinking of it, the Slavic part might not go down so well, but claiming Illyrian and Greek descent will earn Venice some credit in cultural circles. And that's where the main battle of Venetian independence will be fought--in cultural and literary circles of intelligentsia, in order to get Venetian nationalism established. Once its in place, any unified Italy will have to think twice before moving against them, since the public support will mostly be on the side of the "Venetianists" and not any Italian irredentists.
I personnaly dont like Crowning a Royal from an existing familly firstly because a lot of the time they arent descendant in the male line of the first encstor of the familly to many bastard there was king who werent the son of their father a lot of them and even if there was no bastard down the line which is extremely unlikely the british fench austrian ect got bastrad down the line then they are descendent of people who commited war crime and other such crime and a lot of royal who are supossed great for some reform or other thing didnt do the reform or law or govern they just let other and took credit no if they need a royal familly they need to choose a good Citizen.
Ancestry, bastard status wouldn't matter if the royal was any good. But that's the issue with monarchy--too much randomness in who rules, and the royals of Europe could pick a fool to rule the place (by fool I mean someone with no capacity of ruling, not a literal fool). And there's no need to elevate a new royal family in Europe (like European royalty would even respect that) when you could just go for the tried and true model of a republic. And there were plenty of younger sons, Austrian archdukes, just hanging around with no throne but royal blood.
If Venice had to be revived, they should've written their constitution based on the US model, and allowed for a strong Doge (if they even call it that, as I noted earlier--it could just be "President") with separation of powers. The old Republic of Venice is dead--so now they can have the Second Venetian Republic. A strong executive branch seems in line with the traditions of Venice, but now the Doge would have to re-elected every 4 years (or however many--no automatically serving for life). I'd assume Venice would have no term limits (at first, at least) to align with that serving for life tradition.
I say the US Constitution because I imagine Venice would be a presidential republic, but the Venetian Constitution could end up being its own unique thing and in of itself be a model for other countries. If independence is delayed until the 1820s, Latin American constitutions might provide a model/inspiration too.