I recently had a long layover in Singapore in Nov transferring from an SQ to QF flight, layover approx 12 hours. You can do online check in for Qantas so once you bag drop you will have a boarding pass to get through security.
Option 1
On arrival don't collect your luggage, stay on the airside and do what you need to to kill time - if you have lounge access or priority pass access even better. When you're ready to immigrate you'll need to go to SATS lost & found luggage in the same terminal you arrived in. Process is super quick and easy. Grab bags and immigrate. Qantas from memory opened up bag drop for our flight ~4 hrs pre flight.
Option 2
Collect bags and immigrate. Early bag drop at Jewel. Go through security with online check in boarding pass. If you have Oneworld lounge access make your way around oneworld lounges around Changi. Only a few are open in the earlier hours and middle of the day so you may need to jump around. If you have plenty of time the Cathay lounge (need to get a shuttle to go there) is well worth it.
