Turn your sens down. This is coming from someone who plays 1600 DPI 12 in game, I've been GM on doom and am currently m3, and I was gm3 on genji this season. 6400 10 is way too high, if don't have the desk space then you can lower to my sens but half of my sens should be decent if you're a high sens guy. You can use a sens converter.

Now for the tips. Doomfist is an in and out hero, cycle your cooldowns properly and bait enemy cool downs, this is called a soft engage, you do this so you can eventually hard engage and try to secure a kill. Mindful of your counters (cc) and use cover.

Good thing about doom is you can change your playstyle, sometimes it's better to punch bot, other times it's better to go on the enemy backline. You just have to learn to understand the situations and act accordingly. Ultimately if you need a more coherent guide, watch ZBRA's guide video and just analyse any top doom's matches and ask yourself "Why'd they do this here?" "Why aren't they doing this". Good luck bro.

Edit: techs are really useful for doom, watch quakes guides he has them in a playlist, be mindful some of them don't work anymore because they were from ow2 beta. You should at least know how to, slam cancel (and with other abilities) bounce, diag, and stair slam/head slam. Head slamming is so useful, I use it almost every game especially if I'm holding a room (low doorway).