I have recently hit 3500+ SR in overwatch and I'm going to share how I did it.
So first thing you have to realize is that your team comp is only slightly important, I've seen to many people who focus too much on team composition and blame their losses on lack of "X" type of characters on the team (Looking at you genji mains). It's important to have a healer but you can still win a game with just a single healer.
The second point I want to go over is your pick, If you team has no healer don't yell at the others to switch. (Swallow your pride and play a healer) You'll win more games if you cooperate. Now by no means does that mean if you've never played a healer to play one because you still have to play characters you're familiar and comfortable playing or you wont perform well.
My third point is communication, I've had hundreds of matches where people are in the voice channel and they just don't talk (until the last 10 seconds to blame someone else for the loss). Using the voice chat is not only fast but its effective to pass information around that your team could use.
I'm by no means an expert just wanted to vent a little bit, take this post with a grain of salt. If you guys have anything you want to add feel free to do so I'd love to hear what you guys think.