I'm liking this!
I understand the drawback of the Sikhism teachings, however, I have mixed feelings. Reducing missionary strength will naturally push the country towards humanism, so you just won't care about this drawback. Especially for CN nations, you just build in some tolerance and then you might have access to a super powerful religion all the time, assuming that the teachings have a somewhat meaningful bonus since you pay monarch points as well. The things I think of right now, to less railroad Sikhism into Humanism, is by tying a local modifier to it. IF you accept Sikhism, every province with Sikhism gives modifier X. So then you have the choice to sacrifice the province modifier (and missionary strength) for teachings, or you go converting provinces and have fewer teachings.
While I generally dislike copying mechanics, I think it works perfectly in the case of Zoroastrism.