The problem with JQ at CNS is if you are connecting I to D, they won't allow you to even re-check-in to the domestic leg until 2 hours before departure, which is the normal rule, but frustrating in this scenario. For example if you arrive on JQ950 out of KIX at 8:30 am and are connecting to JQ15 at 12:30pm, which is sold as one connecting itinerary (KIX-CNS-SYD), you have to hang around with your bags for 2 hours in the lobby of the domestic terminal (even if the Jetstar counters are otherwise open). At least if you could check them in you could pop into town to get breakfast at a beachfront cafe, or something else more interesting than the terminal lobby. When complaining about this in the past I was told it's because Cairns airport doesn't have a large enough bag hold area, but I'm sure that could be arranged if Jetstar would pay for it.
Will be interesting to see if Virgin learns from this or not, given there will be a number of similar itineraries - VA 78 to VA 146 to Sydney has a 4.5 hour stopover at CNS, even longer than JQ's offering from KIX.
I'm not sure what further sales you're expecting, but for dates in Aug '23 I can see Lite fares for $773.50 return right now (from SYD) which is quite competitive - you want it even cheaper??