I am sorry for your loss. My 8.5 year old GSD died suddenly almost exactly two years ago now, and my first thought while reading your story was "hermangiosarcoma". That is what caused my dog's death.
Did your vet mention finding any signs of hermangiosarcoma in the necropsy? Were they able to rule it out?
Again, I am sorry for your loss. It is never easy, but it is always a shock when we lose them so suddenly.