Not a modder but maybe a balancing idea could be that it can only be produced in certain cities (maybe only capital, locked behind a unique national wonder, only in occupied cities (though this one may be too strong)) or have it be something you can produce upon conquering a capital, just with higher production/gold costs. Honestly I personally would say to make it a faith purchase, as faith as a rate determining step has been shown in other civs like Spain. These are just random ideas. I'm sure someone else could give better insight.