A good question.
A local answer.

Take a shorter walk if you also want time by the water; the two activities need separate travel and preparation.

Map the road with daylight, meal and rest breaks; a long cross-island transfer leaves less room for sightseeing than the map suggests.

Check ferry, admission and car pickup times before assigning the attractions to specific days.

Photograph the vehicle at collection and keep the branch contact and return instructions available offline.

Keep time for Port Arthur itself rather than adding too many sights on the transfer day.

Leave a full day for the transfer between bases if you want a relaxed stop rather than a rushed drive.

Ask wineries about current bookings and whether they can suggest a local driver or transfer service for the whole outing.

A car offers flexibility, but only if you are comfortable with the roads and the timing after your flight.

Choose the overnight stop after estimating the drive from your Cradle Mountain accommodation to the next day's Bay of Fires base.

If Hobart city days do not require the car, compare a later pickup with the cost of transfers for those days.

Check opening hours before walking over, especially outside peak lunch or dinner times.

If the car is only needed later, price collecting it after the walkable Hobart days.