...The biggest issue I've read with the other festoon style LEDs were that the circuit boards were being exposed to the elements and that's what ends up killing them off. ...]
In the stock license plate fixtures?
They are well sealed against the elements, so much so that the LED killer
is heat buildup from both lack of ventilation and the 14.2V supplied to those fixtures.
Many LEDs rated 12V don't hold up well to that combination.
I suspect the length of time for each use has an impact on longevity,
I tend to see an average of 90 minutes each use with lots of 4+ hour cycles,
so maybe that accounts for the short festoon LED life I had seen in that location.
Went through four sets of festoon replacement LED bulbs,
none of the failures were related to moisture, all were heat related.
(and in most cases the failure was strobing rather than no light,
which is worse in my opinion as the rapid flashing draws LEO attention. )
I did get reasonable life from a set of 42dd 14V bulbs, but even those eventually failed.
IMO, an LED replacement should last the life of the car,
and that has been true for every other LED I have used on the car,
including over 190,000 mi on LED tails with no failures.
Good luck on the festoon replacements, but don't be too surprised if you are replacing them again soon.