The TRS scheme does seem to indicate that the goods could be checked through from Adelaide as long as sighted by Customs before being checked in, so would seem that what the OP wishes to do is viable and just to allow time in Sydney for processing the refund

2 hours connection should be enough time to process the refund - I would suggest taking the seamless transfer rather than the train. The train may be more frequent , but is a longer walk to get to at Sydney Domestic