Using the seat map can indicate when a flight is very full, but an unallocated seat map does not necessarily indicate that the flight is lightly sold. The blocked seats are likely due to your QFF status.
The Flight Availability tab is going to be more useful for requesting seats. The particular flight you are looking at (2APR25 MEL-CGK QF39) has unusual availability this far out - only Flex Business (J) and Flex/Semi-Flex Economy (YBHKM) inventory is available.
Qantas may be expecting high demand for this flight and is not releasing any sale inventory in any cabin. VIC school holidays start on 4 April, I don't know if there are any major events in Jakarta/Indonesia around that time?
You could try request 30MAR25 if that date works for you.
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Flying via SYD on 31MAR25 could also be requested.
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