I'm insanely jealous that you have a facility with > 5000 Duranium available.
I wish I had your problems
my newbie AAR total production of Duranium is less than 2000 per year.
Hmm, started a new game.
With a stockpile of 21500 I tried a conditional order to load 10t into an a fleet of 2 freighters (50,000 free) with no result after a series of 30 day increments.
Then, to check to see if only production over the amount entered into the action field I set a 'Load Duranium at 21850' and incremented in 30 day jumps until my stockpile passed over the threshold.
Nothing loaded.
So, if greater minds than ours can comment, I too would be grateful.
Matt.