@Boss Savage After re-reading the OP and the differentiation between the difficulty for picking up a character to play and master at a competitive level, I'm not sure where the latter would be for Venomous. I really don't think it's competitive at all. She could very well be bottom 5 in the game when played at the highest level of competition, and the only thing possibly keeping her from it is universal D'vorah normals are pretty good.
I guess that doesn't really matter though. We're looking at difficulty to master, not whether the variation is actually competitive though. Her game plan just doesn't have any layers. She used to have some armor breaking setups, but the patch killed those. The basic level of casual play is pretty much mastered Venomous. She just doesn't have enough tools for anything else.
The only thing that might pose a problem in mastery is truly optimizing your damage is difficult to say the least. It's not a question of simply knowing the optimal combos, which are for the most part also difficult, but knowing how many poison stacks are on your opponent when you started the combo, since it can be much more damaging to do something different if the first poison period expires mid combo. Ideally, you want the last few ticks to happen outside the combo, where they aren't scaled. I don't know anyone that does this, honestly. More trouble than it's worth, frankly.