I feel your pain.
So, when my wife ran a Girl Scout troop for a dozen years, both daughters using my down bags way more than they were probably intended, I simply followed the directions on my North Face bags.
Wash warm water, liquid detergent, tumble dry.
I use UNSCENTED liquid detergent, warm/hot setting, dry until dry.
My favorite bag is almost 30 years old and good as new. I'll usually zip-tie a sock over the zippers to keep from beating up my dryer. Down is tough. Nothing man-made gets close.
Method: Keep bag in stuff sack. Fill washer and add detergent. Stop machine, and submerge stuff sack...slowly pulling out the bag, allowing it to saturate slowly. Else you'll be wrestling an anaconda trying to push out all the air. Run machine, regular cycle, spin, and spin again. Pop into dryer, maybe a tennis ball or two to break up down clumps, and dry until dry. I've tried being gentle, tried using gentle down specific concoctions, but in the end, normal fabric detergent works fine.
You can also spot clean. I sometimes wipe down my bags by draping them over a taught line, and with a big sponge, wipe them with a dilute solution of Dawn, and clear, warm water. Sometimes the grime never makes it past the fabric. Hope this helps.