I don't do a lot of foraging anymore. No particular reason. If I come across stuff I recognize I'll pick it where legal. But, otherwise, the only thing I actually target are morels. Fiddleheads usually emerge at about the same time, here, so we get to double dip.
Sumac was the basis of a lemonade-like drink in colonial days, which is probably what you are making. But you can also grind the dried berries and use them to provide a citrus-like flavor to foods. It's very big in Mid-east cookery, for instance, both alone and as an ingredient in Zataar.
When I was a kid we used to forage a plant called Indian Pipe. I've never seen it anywhere except in the Northeast, though, so don't know how common it is. It's a grayish-white plant. Stems grow straight up about six inches, then bend into a bowl-like growth. Whole thing looks like a pipe. It cooks like asparagus, but has a more woodsy taste.
I share your opinion of milkweed. But you have to pick them young. Ditto fiddleheads. Once the leaves start to form they're all but inedible.
Down here there are all sorts of edible wild greens, and many people, particularly older country folks, forage for them. They're among the first greenery to appear in the spring, and bring a special flavor to the table.
Kentucky is probably the heart of wild yellowroot and ginsang harvesting. So much so that wild stocks appear to be in danger, and the state is studying the situation right now, and will likely start issueing management rules in the not too distant future.
They have taken the oath of the brother in blood, in leavened bread and salt. Rudyard Kipling