At Jetstar, we do hire a lot of Rex pilots and perhaps to a lesser degree many Qlink pilots. There are pathways to transfer (I use the term loosely) within the group but if you're going to bother doing this, particularly right now, you're better placed to aim for mainline than Jetstar. I say this as a Jetstar captain, not because Jetstar is not a great place to work, I can assure all those interested that it is, nevertheless, Qantas currently is a much better prospect for those aspiring to get in the left seat one day.
Nobody has a crystal ball but the average age of a captain at Jetstar versus the average age of a captain at mainline might be a difference of perhaps ten years. In addition to that, the pay at least little better on the narrow body fleet and appreciably better on other aircraft. Qantas simply has more options for those that get bored, albeit Jetstar has more narrow body base options.
If you do end up in Jetstar, have no fear at all, life will be very good however my guess is that a move to the left seat of a narrow body will be approaching fifteen years by the time the music stops, even if Qantas does no further expansion, by sheer weight of impending retirements, there is likely to be many more commands coming up over the same period of time.
Good luck either way.