The 30 Piety per month (Religion 11) is an optional short cut rule for those who do not wish to use the piety gain table.
Irregardless of how much piety a character may be entitles to it is still up to the GM to regulate it, since it is the gods whim to give piety so to is it the GM's.
How that piety is used is also up to the GM, such as invocation piety cost, piety spent to learn a ritual invocation, sinful acts, etc.
To prevent priests from being walking hospitals you could make the piety cost of healing rituals require double piety if the patient is not of the same faith (triple if of an opposing faith
) and/or have the patient lose some piety for the task as well.
Another way to have rituals not always succeed (but still cost piety) is if the reduction of piety used in the ritual (for either the priest or patient) would reduce the character below the level for divine grace (just a suggestion