My strategy for venice is somewhat counter intuitive compared to the main course of action.
1) only puppet city states that are close to your city a maximum of 10 turns should be the farthest. the landlocked city sates should be able to have their border connect to you. so on an so forth.
2) I actually can go as a bit of a warmonger against neighboring civs just taking surronding cities and puppeting them. the UA applies to all puppet cities hence you can get a lot of great merchants. Use these great merchants to make colonia this can create a incredible amount of food, and gold for venice.
3) get tradition , then statecraft, then finally industry.
This was my strategy up to immortal. ( i actually got a 100 pop venice once but i forgot to screen shot it