Yeah, 10% crystal for a sweet stout is better, I think that would be drinkable. And dropping the c40 is best, thats just going to add mostly just sweetness and body in a stout, and not much else. You could drop the mash temp as well to produce less dextrins, but you know what you like. If you dont mind a chewy slightly heavy stout, leave it, those can be nice in the winter. I mean, we certainly aint making guiness, or beamish, here.
Adding brown sugar later will just add more alcohol. It wont dry it out per se. I mean it will be dry for a 7% stout, but its going to have the same body as your 6% stout if you didnt add sugar.