I would not trim more than down to 1/4" off a flat's fat cap. You need that fat there. If you are cooking just flats, that is the only fat on the meat.
Cooking to internal 200F, that may, or may not be cooked enough. As for smoke ring, the comment about how clean a keg burns in the answer. If you simply must have the smoke ring, put the meat on while the cooker is cooler, around 215F, and let the temperature rise up, the cooler cooker will be burning less efficiently, you will get a little more ring that way.
I'm feeling bearish, and I'm packing a Wusthof Grand Brisket slicer from MABA
"perhaps...but then again...maybe not..."