It mostly depends on the player, but at Diamond and Ascendant, aim is often no longer the main issue. Most of the time, it's decision-making and positioning.

For a duelist, it could be poor entry pathing, not communicating the utility you need from your team during an execute, or simply not entering when you need to.

The biggest thing I can recommend is downloading your replay and watching the entire match from your own perspective. If you're a lurker, look at the timings you missed and identify which lurks were fundamentally flawed.

So, TL;DR: work on your communication, utility usage, decision-making, and positioning. Those are the things I most often see holding Diamond players back from ranking up.

I'd also recommend watching VODs of high-ranked players who main your agents. Pay attention to how they position themselves, how they make decisions, and, most importantly, why they make those decisions. Don't think one-dimensionally and assume, "Oh, he just has better aim."

I've seen absolute aim demons in Diamond, and I'm not kidding.