Okay, got a runaway win with Venice on Immortal (Archipelago), a completely runaway victory. May actually try Deity now.
Scrap culture, scrap science: Venice is made for Diplomatic.
I'm amazed i took so long to understand that.
Wall o' text incoming.
Culture order: Tradition (full) -> Patronage -> Rationalism (first and +2sci per specialist) -> Finish patronage -> Ideology.
Unlocking Commerce (for the Big Ben, or actually going to get the -25% cost) can be a good idea.
Unlocking Aesthetics can be too, +25% on the three Artists can be huge for your culture potential, and for Tourism when you'll have to defend your Freedom teeth and nail.
Religion: unless anything good is there, try to start with Fertility Rites (But if you have 3+ sea resources, get God of the Sea), then head for the +15% production and +15% food. As complementary, Tithe and the +25% pressure can be useful.
Coastal start is a need. Fish is good, damn good, since you're rushing optics. Calendar\Trapping lux are terrible, if both are that way.. consider a reroll.
Build Order is usually (on archipelago) Monument -> Shrine -> Granary, since there's no need for scouts, or workers (steal it!)
Since you usually get the best roll in the world, Abundant\Legendary starts are actually harder.
Tech priority is Optics rush (Step by for a lux tech, if you REALLY have to)
Steal your first from a CS, possibly not the one you're going to buy, but it's still okay.
Get going into full Tradition, leaving Garrison for last.
With your first Merchant of Venice, buy a city state that's in sea range. Get as soon as possible a sea trade route for +7 food to Venice.
{Note: Puppeted cities have terrible cultural growth, so until you slap atleast a great work per puppet, it's useless to give them permanent food trade routes: they'll actually end up with unemployed citiziens}
After that, rush to Currency (if you've got a Desert start), else decide if you want to rush Colossus (Iron Working) or just go for National College as soon as possible.
If you don't rush to currency, still remember that it has the first market specialist, and is critical to acquiring more Merchants of Venice.
The importance of National College, and atleast 1-2 Great Scientists, cannot be overstressed. Get it, and get it fast.
(You should really grab Porcelain Tower, possibly by doing Leaning Tower of Pisa and using it's Great Engineer for it)
Colossus, and/or petra, since they give +2 trade routes instead of +1, are HUGE. They're hard to catch, though. If you've spotted Ramses, don't even try.
After that, catch up a bit in cultural techs (Engineering is high priority, due to +2 trade route and getting those free Acqueducts), and head straight for Compass\Education. Done these two, it's just a matter of going forward, and you can actually start trying to get Wonders, since Venice will be huge.
At this point, Notre Dame is usually a good shot.
Going past 4 cities is pretty useless, since an allied CS will usually provide the same science, if you count the +5% per puppet.
As soon as your gold base is decent, Cultural CSs are VITAL. You'll often have 9-20 culture by yourself, until you start getting wonders, so having +12 per CS means a pretty fast culture growth. You'll be screwed without them.
After Cultural, get Mercantile if you need the happiness, and Militaristic. You need units only to have enough military score not to get a war on your hands, and they're usually enough.
And that gets us to the main point: Patronage.
Pledge to Protect + Consulates means free friendship.
Scholasticism is huge, and while Merchant Confederacy is pretty useless, the finisher is incredible, since you can hold ALL CSs of the game, with Venice's huge gold potential, Patronage and, in lategame, Freedom.
After 3 tenets in Freedom (or five, if gold isn't enough to keep a strong grip over all CSs and you need , finish up Rationalism, get a free tech, blow all your faith on GSs, and rush into Telecommunications (and possibly, Globalization) to unlock the World Leader proposal. Free Tech + 2x GS (3, possibly, if you've got some good faith wonders \ religious CSs) means you can get Ecology+Telecommunications+Globalization all in one shot. In the best case, this should be doable by turn 200. (But expect 300-350, unless you're a deity player)
Oh god why i've written that much. Ohwell.