I really like the Starclass JetStar flight from MEL-BKK. The flight times are great: 2.30pm to 7.50pm. However the latest flight from BKK-USM is 9.00pm, so unfortunately your Jet* flight it will be too late to connect. Similarly flying to coughet will be too late to connect.
So.....even though I feel Starclass is way better than Thai Y, on your route, I would either fly:
A. Thai SYD-BKK on the 11.00am-4.30pm flight and connect with a flight to USM.
Alternatively, I would fly Emirates Y direct from SYD-BKK on the 7.50pm-12.55am flight and connect with the first 6.00am flight from BKK to USM (if you can handle a few hours at BKK airport in the middle of the night.
One observation on StarClass, the legroom is tight, so I would still opt for an aisle seat, Otherwise you will still need to bother your neighbour to get into the aisle (unless in row 1)