Thoughts...
Old Mandurah Rd south from Mandurah is a better ride than the Forest HWY (but its only about 50k's long before joining Forest Hwy again) and would add 1/2hr to the trip by getting off the freeway.
Sue's rd off the Bussel Hwy just after Capel is also a good quality then right onto Rosa Brook rd (actually think its called something different at the Sue's rd end) would bring us out at the southern end of Margaret Rvr. back up Caves rd through Yallingup and then home....