I mentioned this before and I'll reiterate: the Doge's Palace is not a UB. You never choose to build it. You don't get to choose when, where, how, or what to build it instead of. You don't tech to it, you don't build multiples of it, you don't rush it, you don't do anything to it. Its a UA that only applies to their capital. (I feel similarly about the American Hermitage, but at least you have to build the thing and pick a city)
In fact, Venice wouldn't do much with ANY UB at all, since the most you can do with a UB is invest in it in your puppets. What exciting gameplay.
The best thing for Venice is to get a UI. What I suggested before was a sort of trading post that claims tiles around it, can be built adjacent to your territory, and cannot be built adjacent to itself. This lets Venice claim luxury and strategic resources around it that the AI just refuses to settle. Nothing is more frustrating than being unable to make use of land at all because you can't settle it and nobody else is bothering.
Alternatively, make it buildable on any unclaimed coast that ISNT adjacent to your territory, and if possible let newly founded cities steal its land as well as purchase the land at a high cost like the American UA.
I'm listening. Unfortunately, we'd need art for this, which we don't have (and unlike my middling icon art, I can't even begin to do 3d art).
G