Can we take a scenic route from Adelaide to Hahndorf through Stirling?

If someone in the car dislikes winding roads, use the faster route one way and the scenic route the other. That preserves the Hills experience without turning the whole day into a driving exercise.

Keep the day loose: one village coffee stop, a garden or lookout, then Hahndorf is enough. Narrow roads and bends take more attention than the freeway, especially in wet or foggy weather.

Yes. Treat Stirling as a deliberate stop and use the smaller Hills roads for part of the trip. A route through Norton Summit, Summertown or Uraidla can be attractive, but check road conditions and navigation before leaving the main road.