A good question.
A local answer.

Leave slack for weather and a slower scenic drive instead of treating every lookout as a timed appointment.

Compare time spent at the coast with time spent on secondary stops before booking.

Price the overnight parking against the value of an early start and the rental's pickup hours.

Treat them as separate day trips if possible; the driving leaves little time at either place.

Choose the farthest essential stop before fixing the overnight; Apollo Bay is a base, not a promise that every landmark fits.

Two or three overnights can reduce long days, but choose towns that fit the attractions and onward drive.

Keep the final night or return drive generous enough that a delay does not threaten the onward flight.

Finish rural driving before dark where possible and check the forecast for exposed coastal walks.

If an iconic distant stop creates hours of backtracking, add one overnight or leave it for another trip.

Allow daylight for coastal roads and rest between walks.

Map the first coastal stop and daylight before choosing a pickup location merely for convenience.

Give the coastal section enough nights for stops and weather changes instead of judging it only by the fastest map time.