There is a option missing here, related to the size of the vehicle you use
We have a golden retriever, weighing in at about 90 pounds. Our daughter has a second one, they two of them occupy the entire back seat of our 4x4
Actually since these dogs have never been crated the only way is to leave them unrestrained. In my experience, large dogs will be of one or two types, they either lay down and sleep, for which any passive constraint is not really possible, or they sit and want to look out the window. Ours is a “sitter” our daughters is a “sleeper” when going on long trips.
Short trips to the park, they both sit.
If people are going to passively restrain them, the only way is a full harness for the dog, so that in an accident, any forces from the restraint are on the body not the neck.