You could also look at taking a day room at the ridges (next to terminal) or any of the other hotels, or even a Sydney hotel, as it is very easy with the train. You could take the train to the domestic QF J lounge - it will be a flaff though.

There is no way to backtrack from the F lounge through immigration. I once did it when a flight was canx and rebooked on a domestic connection to a new intl flight. And I had to be walked/escorted and manually processed.

The lounge will have no problem if you want to sit there for 8 hours, many people do it, esp in SYD. But I would rather go into town and get some lunch etc.