What is the correct temperature for storing home-made frozen yogurt?
-18°C (-0.4°F) is not too low for storage (as this would be the normal temperature of a household freezer, where I store my ice cream and frozen yogurt. I definitely wouldn't set the temperature higher if I store also other stuff in the same freezer).

For serving you would probably want to let it sit out a little bit, so that it softens up a little so that you can scoop it (same as with ice cream).

If it is way too solid, you maybe let it thaw too much before storing.


Then, you can fix the texture by re-blending it in a mixer (you can get even more ideas if you search for recipes on "how to make a frozen yogurt/ice cream without an ice cream maker" - often it involves freezing in an ice cube tray for easier blending).
Or by thawing it again and using an ice-cream maker to get the desired texture.