Why is my Frozen Yogurt Grainy?
Quicker freezing = smaller ice crystals.  One hour is a long time, that likely contributed to your grainy texture.  If the Breville ice maker is your only option, make sure your base is as cold as possible before using the ice cream maker.  That means refrigerating it for several hours in advance.  You also might want to strain your base through a fine sieve first.  This would help to ensure that the graininess is not coming from your ingredients.