See photo for review.
Our take:
Probably easier / faster to drive to Bunbury / Busselton via highway passing by Mandurah - skip inner road to south via Harvey.Loop to Margaret River via Cave Road / Dunsborough which is more scenic.
https://www.australia.com/en/itineraries/perth-and-surrounds/14-days-driving-australias-stunning-south-west-corner.html
https://www.westernaustralia.com/en/things_to_do/must_do_and_top_10/pages/top_10_free_things_to_do_in_australias_south_west.aspx#/