Homebrewing - Beer, Mead, Wine, Cider

Thanks for sharing. Mind if I ask why you used flaked barley and not just more base malt? By black barley do you mean roasted barley (unmalted) or black patent malt? Assuming the unmalted sort here, but just to make sure.

Your mouthfeel lacking could be from the mash schedule. I usually go around this with some crystal malts to just bump the body (go lighter or darker depending on your desired flavour profile, too). For stouts generally, I mix up black malt and roasted barley with a bit of crystal and that gets me where I want to be flavour wise and mouthfeel wise.

I will admit to never having used flaked barley, so no idea what it'd do there. I'll also admit that my mind goes to Guinness when thinking Irish stout. But that one's fizzed up with nitrogen, not carbon dioxide, hence the smoother mouthfeel. If you want to try Guinness with CO2, look for Guinness original or something of the sort, bottles or cans without the widget in them. That tastes and feels extremely different to the nitro one.

As for what to do to it now, you could soak some coffee beans or cacao nibs in whiskey and plop them in once properly sanitized by the alcohol, see what that gets you, but if it's too dry, there's not much you can do to add body that I can think of. You can also try to brew some strong coffee and add that to your brew - bonus, if you add it hot, it's already sanitary (this is the route I'd go rather than using coffee beans, way easier and no floaters in my beer). I've used moka pot brewed coffee before - something like 200 mL (a mug full) for a 20 L (5 gal?) batch of beer was noticeable. You can probably double that if you really love coffee stouts.