One thing you should definitely not do is make it illegal but then condone the usage and distribution of it but not the other parts of the production of softdrugs. This is a major issue in the Netherlands where coffeeshops have to buy the drugs from criminals because growing it is still illegal whereas selling it to users isn't. This also means that it still cannot be taxed and that coffeeshops themselves cannot be taxed properly either since they can hadrly keep book on illegal transactions.
All in all, I say, just legalize the whole cycle of production and put a gigantic tax on it. Besides that make the usage of it only legal at home or in coffeeshops with special licences (and not on the street/train station).
Just to add, harddrugs should remain illegal. IMHO.