But not able to use before the outbound departure
Not true actually, it is quite legal to use discretion at any point after reporting for duty. You might very well decline to do so (as I have a few times) but there is nothing in the regulations that says you can't do it.
The problem with discretion is that it is supposed to be for unforeseen circumstances that occur on the day, my view is that if you roster a crew to within a few minutes of max FDP then it is not exactly unforeseen that you are going to run out of time. I have seen duties that involved 4 sectors in and out of a busy airfield rostered to within 10 mins of max FDP, is it any surprise that the crew run out of hours? Not in my opinion and I declined to take the crew into discretion on that basis.