Someone like @Mwenenzi will be along soon with a better answer, so treat this as a bit of a placeholder

i don’t think there is an arrivals lounge. I also don’t think there are any paid for lounges once you’re into domestic. There may be restaurants that offer some kind of priority pass benefit? There are also free showers available (BYOT)

if you have 5 hours to kill, the qantas domestic terminal has a couple of interesting exhibits- a replica of the AVRO that was their first plane, and a heritage collection/small museum. Both accessible without a qantas boarding pass and well worth a look