6 is Emperor.
I consider the Colossus as the #1 early game wonder for Venice, unless you have a coastal desert start in which case Petra could be better. If you make a serious effort to get it on Emperor, you will. Venice benefits from the Colossus more than any other civ and it is the best way to bring AI cargo ships your way, especially because you can't settle new cities. On top of that, it generates 5 GPT and 1 GPP towards a MoV.
Hanging Gardens is nice, but I wouldn't bother unless you can't build a regular Garden. Because of how good Venice's starts usually are and the production bottleneck, the likelihood of an early attack by an AI is very high. You probably won't have enough gold to buy an entire army that early on, so you should consider building some units to defend yourself with.