You're absolutely right that the black tank valve must not be left open while parked for the reason you alluded to.
I generally fill the black tank to about 1/4 with fresh water while parked, to increase the percentage of overall water. If you're dry camping and need to stretch the black tank capacity, I would not recommend starting off with 1/4 water.
However the black tank should not be dumped with less than 3/4 capacity, and unless you really need to stretch the tank, use generous amounts of water when flushing to keep the water percentage high.
The key to successful flushing is the whooosh effect when you pull the valve. The whooosh effect is what drives the solids out of your tank.
To help in evaluating the quality of your flush, get a transparent elbow at an RV supply store which is installed at the tank outlet, before you connect your hose. That way you can see what's coming out of the tank.
I believe in a transparent elbow so much, I carry a spare in case the one in use breaks.
If your tanks do not have a rinsing feature installed, get a transparent elbow with a hose adapter so you can backflush your tank. Of course you would not use your drinking water hose for that purpose, but a cheap utility water hose instead.
Do you do more dry camping or are you parked in a campground?