If you can’t train or control the dog, don’t have the dog.
And yes I’ve owned several GSD’s.
I doubt the sign helps you anymore then if you dog bit someone on your property.
The sign is actually a negative. If you go to court, it will be used as proof that you knew the dog was vicious.
I not only have owned a bunch, but have Malinois and Dutch Shepherds and trained many for protection work. I am in the country too, but mine is shut in the house with the door locked. With the Mal I have now, the door lock is to protect the public, I wouldn’t want a friend to open the door.