A good question.
A local answer.

Alternate long tours with shorter city or Fremantle days so early starts do not dominate the holiday.

Check the wind, rain and ferry conditions close to departure. A light shower is different from an unpleasant exposed island day.

Compare total trip cost across several weeks, including accommodation and any school-holiday demand, rather than flight price alone.

Keep outdoor plans flexible and check the forecast each morning. A wet day does not need to cancel the whole trip.

Confirm late arrival food options, current transit fares and road conditions close to travel. Leave a spare day so weather does not force every stop into one drive.

Check separate forecasts for Perth, the island, the south-west and the northern stops just before departure. They are far enough apart that one city forecast will not cover the trip.

Choose one or two regional anchors rather than trying to cover the whole state. Perth, Rottnest and a south-west base can fill a week without constant packing.

Carry a warm layer and rain protection even for urban days. A week gives you enough room to move a booking without losing the whole itinerary.

For an island or coastal day, confirm the ferry or transport timetable and the conditions on the day. Carry water and sun protection; a breeze by the water does not remove sun exposure.