A good question.
A local answer.

Check road conditions and sunset, and avoid promising a long scenic detour immediately after travel.

Reserve several city days and at least a couple of nights for the coastal section. Trying to reach the signature coast and return on one day leaves little time for the walks you value.

Pick the hotel and car pickup together: a simple transfer on arrival and an easy morning collection matter more than being a few kilometres closer to the road.

Confirm child-seat requirements, farm-stay arrival times and any wildlife or beach safety advice before leaving. A spare city day is useful if weather disrupts the coast.

Give the coast at least an overnight stay if you want time for stops rather than a long out-and-back drive. Keep the final night or departure morning close enough to the airport for a comfortable buffer.

An overnight trip gives you room for beach towns, walks and unplanned stops. If you drive, allow for winding roads and do not treat the Twelve Apostles as a quick return trip from the CBD.