1. “Thus” signals some logical conclusion to a drawn argument. The sentence doesn’t do that. The structure can look like: Sitting through lectures --- helps know plant classification --- so botany knowledge expands. So “this” is more apt.
2. I think we need an “are” in the place of “is” - to make the sentence grammatically valid.
Perhaps a better sentence is: I want to study topics, outside the curriculum
Student or Learner