Thought I'd come back with a report on what I experienced.
On check in with AA the agent was initially unsure if she would be able to "short check" the bags to SYD. However she was able to enter a reason - "business meeting" was chosen, and bags were short checked successfully with Oneworld priority tags. BP for QF5 to PER was issued but of course, no D sticker was provided.
Bag emerged in the first batch thanks to priority tag (or luck) and I had nothing to declare so waltzed through customs faster than ever.
However, F check in was unable to accept my bag at ~7am as QF5 check in was not open until ~2. I pushed slightly as I already had a BP, but my question about "what if I had it through checked" didn't help. The rep suggested asking the QF baggage services team if I could leave the bag with them for a few hours - this desk was unattended so I parted with $15 at the nearby storage provider instead.
When I returned in the afternoon, I thought I'd check again in case I choose to do this again. The rep at J check in (was directed there due to a queue at F) confirmed that in all cases, you should short check the bag and clear customs in Sydney, but they wouldn't be able to accept the bag early for QF5 (unlike if I was connecting through to a domestic flight from the domestic terminal). They went as far as to recommend not doing this particular itinerary as dropping the bag early was not possible.
I didn't enquire whether, if doing HLO, it would be possible to go straight through and spend the day in the F lounge (eg. Getting my D sticker on the BP at the transfer desk, or exiting and then going back in straight away). However I did notice that on going through customs, border protection stamped my BP, suggesting that entering (eg. To visit the lounge) and then exiting might be difficult.
The only people I didn't check with were the lounge and the QF international transfer desk (pre-customs), however, based on the two above interactions at F check in I believe the advice provided by many of you in this thread prevails - short check and exit.
Zero regrets as I had a great day in Sydney and the $15 was well worth it due to being able to enjoy the F lounge vs domestic J. But thanks all for your advice and feedback.