Been there, too! :)
Plastic bags aren't water vapor tight in my experience. If you want to keep pellets really dry, I recommend using Mason jars. They're pretty cheap, reusable, and do the trick. Avoid any wood dust on the sealing surfaces by wiping off the lip of the jar before putting the lid in place.
It took a few weeks for our microwave oven to stop stinking even though I scrubbed the surfaces I could reach! ;)
If I use the microwave oven now, I heat for 30 seconds max. Stir. Do it again. Stir, etc., for five or so cycles. That's doing one
AMNPS worth at a time, in a microwave safe bowl.
But lately, I've been heating bigger batches in large sheet pans in a convection oven at 200 degrees for an hour or so. That dries them well. Then I jar them for storage until needed.
A while back, I weighed some pellets before and after drying. It was surprising how much mass they lost. And it's pretty dry around here!
But at our elevation (about 5300 feet), I need all the help I can get to achieve reliable burning.