Pretty good dude. Your aim is pretty decent, you mostly just have to work on your decision making and movement. You have a lot of avoidable deaths simply because you tend to walk in straight lines while shooting (e.g walking directly into a Zarya beam instead of strafing to dodge it) and not using line of sight effectively. I saw a lot of deaths because you ran away in a straight line rather than trying to lose line of sight or simply strafing to make yourself harder to hit. The biggest thing I noticed when I played on my 2nd account (placed 2400) is that headshotting people was incredibly easy because they walk in straight lines. I'd say it's more important to work on aim/movement than anything else, you'll climb faster doing so.
One thing I would say about your ults is that it's pretty important to shoot first before ulting. Don't make the mistake of getting a good flank and ruining it by simply pressing Q. I promise you'll get a lot more big 4-5 man ults if you get 1-2 people low with 3-4 shots before pressing Q. Other than that just try to play with your team more instead of constantly frontlining and trying to get picks. E.g your Mercy on Dorado was getting destroyed by Reaper and nobody seemed to want to help, but you were clearly able to out aim that guy so if nobody else wants to defend your Mercy you should do it instead. On Dorado last point it mostly comes down to either a big push from the attackers, or a pick from the attackers and follow up push. You being in the frontline and getting a pick then dying on defense doesn't actually help very much, keeping your Mercy alive is a lot more important. Although stuff like that you'll just pick up as you get better.
Yeah, I am learning reaper to be honest, in all the time I have played I only notched up 4 hours with him before playing him yesterday. Noticed I was being rather effective with him and thought I would give him a bash.
I get over eager right now and disengage either too late or not at all as I think I can get that extra pick or whatever. It's something I generally pick up in time, but it takes making a lot of mistakes for me to get it.
My ult usage is generally garbage, I know. I underestimate people's use of the Reinhardt shield mostly. Mainly because when I am a support and someone does a death blossom etc. I generally found that they were more concerned with themselves than the others. I have started more tactical use of the ults since them games, but I still have a lot of work on working out the perfect time to use it.
As for the front lining a lot. Yes, that is a thing that I need to chill on, but I get overeager to take out key characters and becomes almost an obsession. I will get those things down in time, it does take me a while to stop doing it though as I am fighting the natural instinct I have to be aggressive. Which is born from other shooters and an unhealthy amount of time playing solo queue quick play.
The good thing is, I generally see the mistakes I make. The bad thing is, me trying to fight my natural instincts take me a lot of time. There are other characters I am a lot more solid tactically, but to be honest I don't make quite as much of an impact on the games then. So I will keep working on it. Just have to make sure I don't go from being too aggressive to too passive as when I immediately try to alter how I play. I often go from one extreme to the other.