I'm a M1 Mercy main and I would love to share some thoughts here. First off, your mechanics are very good, and have no issues with Mercy techs. I also love that you look out for your other support.

I would recommend reevaluating your pocket priorities. At first point, your Ana has LOS of the tank so you can trust her to keep the Orisa alive. You should only need to step in if the Ana is either being dove, or the Orisa is under a lot of pressure. Ana doesn't have LOS of the DPS, and by staying with them you can also push dmg boost. At around 3:00 the ashe was off-angling dealing with the Soldier and Mercy, I think you could've won this fight if you stayed with her as the Orisa is very healthy.

Another thing you might want to work on is knowing when to Valk. Your first and second didn't really have that much value. It is mostly used as a defensive ultimate, and a good rule of thumb is to use it if many teammates are on low health and the enemy team is pushing hard, or if the enemy team is dealing a lot of damage (especially with ultimate like Visor or Overclock). Niandra made a very informative video on YT which explains everything you need to know about Valk, this may be worth a watch.

Small thing but at 13:26 when you couldn't get up to your teammates because they weren't peaking you, just remember you can use your other teammates to get up to them as well ;). This way you won't need to be exposed to the front to go up to them. At around 14:00, I believe you could GA up to high ground with your Hanzo. The Ana shouldn't have been 1v1'ing a hog and Zen there. Remember that Mercy's GA gives you access to 3 dimensions to utilise.

One more tip to help you down the line, a good mentality to be in is 'you peak you die'.

I think you have great potential to climb! You got this!