You can use Tween's signal
One way of implementing this is to keep track of the number of sprites that exist and the number of signals that have been called. When the number of sprites is equal to the number of signals called, it means that all sprites have completed their tweens; assuming that no sprites are deleted, tweens are not set to
repeat, there's only one sprite per tween and one
targeting_* * handled per tween.
*I couldn't get these two to work, but they should also call the signal when completed, thus if you are using more than one of the those three functions per tween, you'll have to keep track of that when updating your
number_of_signals_called variable because the number of signals called will be higher than the number of sprites.