Hi Jordan,
Yes, this unassuming kettle is capable of many different sophisticated methods. For that reason, there's no "recipe" for a guarantee. That said, it's NOT finicky, either. So relax!
And I agree you have the basic concept of the airflow down ... the bottom vents are primarily intake and the top, exhaust. Stick with that idea for now, or for a long time. Less temp? Close down the intake for a awhile. Open back up if temps get too low. Wait 10-15 mins for the temp to settle, but settle it will.
As buckie shows, indirect cooking gives you power and control. You may find you do a series of cooks with too-little heat, but that's overcome with using "too much" (or so it may seem ...) charcoal. Turn down the heat via the bottom intake. And since you're closing everything off at the end, right? you've saved charcoal for next time. A Weber chimney is needed, as is better charcoal. We have threads here about chimneys ...
Much of that "BBQ" (charcoal) flavor comes indirectly. Consider: air comes in from the bottom, heat flows from coals on one side, goes OVER the food and out the top vent positioned opposite the coals. Picture it?
You can certainly do a hot, direct cook like you did, but fish I'd turn at about 4-6 mins under such conditions and remove at about 8-9 mins (like an oven broil?) , so I'm guessing you overcooked by double because it was direct + left on too long.
Might not have been piping hot if dried out ... one of ironies of meat that's lost juiciness from overcooking.