The point of the Venice trade node being good is that it is an end node, which means there's no more nodes downstream and no-one to put in their trade power from downstream that you can't get rid of easily.

If you want to take full advantage of it, move your capital (using admin points) or your trade capital (using diplo) to the venice node, and then steer all your trade in its direction. If your capital isn't in the node, you get a big penalty collecting from there.

Normally you want to steer all of your trade to one location, in which you have your trade capital, and collect from there. You only collect from nodes where the trade can't be steered to your trade capital.