There already is the notion of more leniency for the competition between syncretic religions like Confucianism/Taoism or Hinduism/Buddhism, for example in the stability code, religion unhappiness or religion spread chance (i.e. Confucianism does not count as another religion already being in the city to reduce the chances of Taoism spreading).
The same concept can easily be applied to the piety calculations. In fact, once piety is in place and factors these things in, all those arbitrary extra rules in the other systems could be removed and rely directly on piety instead! For example, instead of Hindu and Buddhist civs not disliking each other, Hindus would not dislike non-Hindus with high Hindu piety. Since Buddhism tolerates Hinduism, keeping high Hindu piety is easy for Buddhist civs and so on.
For civs like China this means that going Confucian/Taoist is the tall option where you choose your state religion for the preferred bonuses. Adding Buddhism is the hybrid option (which should also be viable, although probably only with high religious building density and without access to the best religious powers). Throwing in Christianity as many Chinese strategies do right now would be the wide option.