The last time around the stacks did not replenish so the ideal case would be to do them at the very end since you can only do it once anyway.
That said the points totals are pretty different from last time, so I wouldn’t be surprised if something else is tweaked behind the scenes. In fact, I wouldn’t be surprised if the missions are simply not repeatable this time around.
It won’t recharge. If the person in first place completes his missions quickly, then it doesn’t matter when you do your missions, since rubberbanding will never change. Even then, the amount of rubberbanding you’ll get by waiting will be fairly small.
You would only gain a few points more due to rubberbanding, which probably takes action earlier than the tournament end. Waiting 2 hrs probably maximises the effect, for this part of the tournament. Also, if you do them earlier, u can try to do the hard misssions more times.