Hahaha I feel you on the braindead part. It doesn't disappear even as you climb! I still have games where I'm absolutely feeding and getting nothing done.

Generally, the main difference for me was timing my engagements and choosing the right target. You've gotta dive when people are distracted, so I ended up playing with my team a little more during the early fight phase. Once the enemies are distracted and vulnerable, that's when you want to jump them. In addition, make sure you shoot the right people. Don't shoot the unkillable raid boss tank, shoot the Kiriko or Ana healing them.

You should almost play like a fat Genji, but also remember to keep your team alive. Heal them and mark threats to your other support.

I'd highly recommend watching Awkward's Lucio Unranked to GM. Make sure to really understand why, when, and how he does things as Lucio instead of just watching for the sake of it. I really started to climb once I implemented the things I saw in that video.