I have read somewhere (maybe The Perfect Scoop? But I don't have the book here to check) that the best storage temperature for home-made ice creams and other frozen desserts is -6°C. And I have the same problem as you, with homemade desserts being generally too hard, even without the presence of noticeable crystals. They just don't have as much overrun and dry matter as the store-bought ones, which are formulated for colder storage.
As Martin says, keeping other stuff at -6°C will affect the storage time of other products stored in the same freezer. If you don't have a freezer with separate compartments, you will have to decide between keeping it at ice-cream-friendly temperature or using it mainly for long term storage.