Consider another Sheltie...perfect sized dog as I see it.
Or a Lab. I've grown very enamored of these dogs.
When they wag their tail the entire bottom half of their body moves.
We had Lassie from a puppy and she was so different to the 2 we had a grown dogs though Ziggy was not that nervous he was not the adventurous type that Lassie was. Bluebell was so nervous that if this was your first experience of the breed you would never have had another one.
We had a friend who had one from a pup and that was quite nervous so I think it is a characteristic.
Here is my beautiful ziggy