In Lines of Communication, the Drakh after hearing Delenn's name abandon all pretense of peace and attempt to kill her because she helped end the Shadow War. After they defeat the mothership, they don't seem to continue. This seems inconsistent with the fact that they are tactical and ruthless. Why wouldn't they continue attempting to kill Delenn?
Delenn was attacked in season 5 Movements of Fire and Shadow. Though it is debatable if the centauri drone ship was attacking her specifically or just any ship.
The expanded universe Centauri trilogy provides an additional answer. (There were TWO planet killers which Vir and the technomages destroyed one of. ) But the Drahk's original plan was to attack Minbar with the plague and not Earth which they had planned to destroy completely.
Aside from those examples the Drakh can wait and scheme for decades and decided to use her son instead for their plans.