In my opinion the Hex Dumbbell hold is a lot like a plate pinch with super wide plates. Super wide pinches are stressful! Especially if you already worked those muscles doing plate pinches, then you hit them again, in a super weak position. I'm pretty sure this would make my forearms hurt too.
I would just pick one, plate pinches or hex dumbbell holds, and work that lift for awhile. So, pinch plates for a month, then try hex-dumbbell holds for a month.
In addition to stretching your extensors, you can also work them to keep them strong. I like to do 1 or 2 "grip" exercises, and 1 "forearm" exercise during a grip workout. Like pinch and reverse curl. Or farmer's walk and wrist roller. Hex dumbbell hold and reverse wrist curls super-setted with behind the back wrist curls.
Some guys recommend doing finger extensor work, spreading your hand open against rubber bands or in a bucket of sand or something, but I don't have any experience with this. Doing some wrist work, making sure to hit the muscles on the "top" of the forearm was usually enough to keep my forearms feeling OK.
A sample grip week could be like
A few Gripper squeezes at home
And grippers at home, once or maybe twice a week. I always found it hard to build strength on these things. Usually I would try to progress on plate pinches, wrist work, thick dumbbell stuff, then come back and test myself on the grippers to see if I'd made progress.
Are you just trying to get a stronger grip in general, or do you have some specific grip numbers you want to hit?