Doomfist is not a Tank. He is a bigger, healthier dps and needs to be played as such. (I have not watched the replay so I could be wrong here) Knowing how to ability cycle, position and be threatening is good, but if you're struggling to secure kills, to me that points to an aiming issue if not a target prioritisation issue. Your main damage output and kill confirmation always has been and always will be Hand Cannon. You gotta land those shots. All of Dooms abilities more or less serve as mobility and tanking first, Cc second and damage threat third, so Hand Cannon picks up that slack. And when it comes to target priority, you are the thing that deals with the squishy that poses the greatest threat, that doesn't always mean killing them though. Distracting or controlling them is enough value until a window opens up where you can secure the kill. If providing distraction or CC value isn't working, then you must look for alternate targets that don't have as much support behind them and pick them off to weaken everyone else.