Daoism (in whichever way your interpret it) doesn't place much emphasis on missionary work. As a philosophy, it is something which must be studied and grasped rather than spread. As a folk-religion, it isn't very different in nature to any other ancestral religion.
To recreate its rapid spread in the game, it might be an idea to encourage missionary production in Buddhism. Though this isn't immediately faithful to history (various leaders and scholars have eagerly spread Buddhism, but missionary work in its traditional sense isn't a priority), it should ultimately result in a historically accurate 'range' for Buddhism (which even reached Axum by the 4th century). Though Hinduism could also be found throughout southern Asia in the past, it makes sense to give both Dharmic religions in the game a relatively passive role.
For example, Anekantavada (literally 'non-onesidedness') is a common feature among Dharmic religions even though it's known under different names. For brevity's sake, it teaches people never to impose their beliefs on others or even regard their own as superior. Anekantavada however, is only central to Jainism (it is merely encouraged in Hinduism and Buddhism), which may explain why Jainism is far less popular than Buddhism or Hinduism today.