You are correct, the coast route can be better sometimes. The route marked on the map is a compromise, it connects to many scenic places, and it uses reasonable sealed roads for the complete distance. The route aims to be close to 1000km.
The route follows the east coast, which is the dryer and calmer side of the island. Plenty of good swimming beaches (in warm weather). The road is often right on the coast.
In many other parts there is no road directly on the coast.
In the north-west there is a good road along the coast, a newer highway and an older coast road, from Devonport westwards. But riding on the NW coast misses other scenic spots around Sheffield, the 'Mersey-Forth' area, Cradle Mountain, the 'Link Road', etc.
The road south from Marrawah is not all sealed, some of it is gravel for considerable distances. It is also less inhabited, no shops or other facilities. From Arthur River to Savage River I think is all gravel and all uninhabited, so you are on your own and need to carry all supplies. South of Savage River at Corina at the Pieman River there is no bridge, but there is a punt (little vehicular ferry) that takes vehicles across.
Another possible route is along the coast to Burnie then south on a sealed road down the west coast.
On the west coast the road only reaches the coast at a few isolated spots. One of the good areas is Ocean Beach near Strahan. The south-west corner of the island is a national park and world heritage area and there are no roads except the one that cuts across it.
As our web pages say about this around Tasmania route, it has been selected for "taking in most of the favorite tourist sites as well as some less well-known beauty spots", and also "If you have a little more time then have a look at the recommended side trips off the Giro route (marked on the map in yellow)"
This time of year the weather can vary from great (cool/mild, still and sunny) to terrible (wind, rain and snow). Generally the eastern half of the island is dryer and has better weather, the average annual rainfall is only 300 to 600 mm. But on the west coast it can be wet, some mountains have 3000 mm annual average rainfall, and there is temperate rain forest. Our predominant winds are from the west.
I hope you have a great trip,