It does look like the Heatherbrae bypass could be finished earlier, and in fact the whole project looks like it could be done well before 2028, but we'll see. As for the bypass opening before the rest, from memory back when the Woolgoolga bypass was finished RMS were adamant there were no plans to open that section before the rest of the upgrade despite it being finished, but only a week later it was opened to traffic. Perhaps that might happen here too.
Either way, exciting to see this project going so well after waiting an eternity for it to start in the first place. New South Wales really is a hive of construction activity.