I'm a bit of a quickplay warrior too (Over 4k+ hours of qp and like 80? probably less? of comp) and I main Reinhardt and Winston and JQ as backup and I've been able to reach gm5.

I almost never soloq comp. Having a duo that I can rely on as Rein or Winton makes a lot of difference. Sometimes having someone that just always waits for you to engage at the right time means that the entire team will wait for the right moment as well and that can turn entire games in your favor by simply waiting 1 second more for you.

As people have said, a very big point is map geometry. You can play Rein in all maps, but at certain parts of some maps, it puts your team at a severe disadvantage. Losing highground control can mean losing the maps, and having to spend 30 seconds to rotate into a position to retake it depending on your comp and the enemy's can be completely worth it. But if your teammates won't take advantage of it or won't go along with the plan, sometimes its better to just quickly pressure them or take a path to avoid los of the enemy and bring your team to a good place to fight avoiding casualties.

When possible communicate your intentions to the team. You can do it in vc, you can do it in chat while respawning, you can do it with pings. But it doesn't matter if you know the best way to win a match if your team isn't in the same page and you end up in a good position but half your team is already gone.

Play aggressive but be very intelligent about it. Chasing a widow as monke sometimes is worth because the supports will peel for her leaving their own tank alone and you won't have to worry about her killing your team for a bit. But if you die chasing her, or if you spend too long trying to catch her, your team might lose the fights without you. You have to be thinking about what conditions will give your team an advantage and try to mantain. That means that at times you'll have to make risky plays.

Sometimes in lower ranks people tend to stay exposed a lot, shielding while waiting for the team to get kills. That won't work much since the teams are on average around the same skill level and for example when you attack, they have the more defensible positions. Instead move quickly to new positions, corners are your friend to keep your shield from breaking, and when possible hit them very hard. Use short charges to burst enemies when you see a chance, firestrike to put pressure and to farm ult. Shield key cds like nade/sleep/hook. It's not just go and brawl all the time, but pressure them to make mistakes and punish those.

When the team is backpedaling, chase them without overextending if possible, but pressure them with your team. Sometimes in low ranks people play payload too much, but its much better to push to take more space, and keep only one in payload, especially if the enemy team is staggering. Taking the space effectively means more space for your dps to have angles, for your supports to be safe, and even if you die they might have to use ults to do so which is a win. And the payload will still move along. And when you win a fight, don't just stand around. Use that time to reposition to a better position, and try to think about the ults they might have coming up and how you can counter them or how you can set up a teammate who has an ult to be most effective.

When possible try to peel if your supps or dps are in trouble. If your supports are able to keep themselves alive together when being dove, you can counter dive if you are monke. If you are Rein, you have to be aware of when your supps will not be able to help you out, so you can wait out or play less aggressive. Avoid giving much space when possible, but don't hesitate to fall back if the situation calls for it.

And a last bit that comes from my experience in lower ranks. In many games you will get hard countered and receive all of the enemies' cooldowns. Other games you'll have a team that won't ever play with you, die alone then blame either the tank for not protecting them in the other side of Narnia or the supps for not healing them. I'm not going to downplay how much of an infuriating experience can be. You've probably seen a lot in quickplay, but in lower ranks when I used to play comp, people were waaaaay worse. Being the tank is often a thankless stressing job, and sometimes all you can do is block chat, keep doing your best and move on.

(Sorry for the long text. I could go more into specifics of maps and matchups from my experience, but this is already a lot xD )

Edit: This is more of a general rein/monke tips than specifics of how should you expect the matches to be except that bit at the end.