I've not heard of THR DES before, it doesn't feature on the A320.
What's lacking by just having THR CLB, THR IDLE, SPEED/MACH? They're the only applicable modes I can find that the A/THR will give on the FMA for the situation we're talking about here.
If you're on the descent profile and managed, won't the ATHR just maintain SPEED or MACH like it does on the A320?