You said it. Some years ago I bought a pair of Shelties bc they were so beautiful. They were my crash course on working dogs. Nothing was a game for them; everything was a job. Throw a tennis ball, and it was a death match between the pair, to establish which of them was able to successfully do the job.
Shelties don’t require as much exercise as some breeds, but they do require activity that they can interpret as work. I loved those dogs vastly, but I don’t go in for working breeds any more. Too heavy on the maintenance.
I love shelties but you said it, they’re serious. Rough collies on the other hand are clowns. Those two look alike but don’t act alike.