When I was serious about winning, I’d track the usual stuff but also make note of any “interesting” situations or play lines I saw or made. Stuff like why I thought I won (or lost); if a loss, was it misplay, bad draw, or inadequate answers for the game plan the other deck had? Did I draw cards I couldn’t find a use for? Did my game plan work, and if not, why not? Did my opponent surprise me with something cool I hadn’t seen before?
Perhaps more qualitative than you were looking for, and it might sound onerous, but it really wasn’t. A couple sentences at most :).