For Eastern and Sunstate at least, there is no contract that states a return of service once you join. Some went to mainline in just over 12 months, others took much longer. There were plenty of intakes that had a lot more than 2 former QLink crew. Usually it was at least double that.
There is no way to say exactly if being in the group helps your chances or not. Make your choices based on what matters to you. Most AOC’s in the group would offer a pretty good place to hang out while you applied to mainline. You’ll also get the benefit of exposure to QF Group SOP’s and C&T. Keeping your nose clean in that regard goes a long way in where you ‘rank’ on the hold file.
You can either try your luck in the smaller internal pool or the much bigger external one. One isn’t exactly better or worse than the other.