And if CTAF’s just about coordinating circuit traffic, why do I have to broadcast on the CTAF if I’m overflying at 3,500’?
I said "at this type of strip" i.e. in this example, your own private strip. You broadcast overflying a regular CTAF because you might conflict with other aircraft at the airport, e.g. an instrument approach that might begin a few thousand feet above. That's different from a private strip somewhere.
If the broadcast at a private strip is not primarily about conflict during takeoff and landing, why would you broadcast when you are landing at your private strip, but not if you e.g. orbit a neighbors house without a strip?
Ultimately you need to figure out whether you are likely to conflict with other traffic, and if so, what frequency that traffic is most likely to be on. If you expect to conflict with Mildura traffic, Mildura CTAF would seem to be the most likely frequency. If you are too far away from Mildura for the frequency to be applicable, how likely are you to conflict with the traffic, really?
For better or worse, it was decided a long time ago that details of VFR movements should not, in general, be broadcast on the area frequency.