tl;dr edit: "complexity" represents how difficult the task is, while "urgency/importance" represents how much achieving the goal means to you.
One thing that I've found helps me conceptualize my promises when I'm playing is to reframe "urgency" as "importance". It's pretty much an identical concept, and unlike adding something like "scale" it doesn't overlap with the idea of complexity. A big task means it's got more things to do, which I would say means a higher complexity score. But an important task doesn't say anything about how much there is to do, just how much completing that goal is weighing on you.
This works for me for two reasons: one, literally all I need to do is change "urgent" to "important". "laid back" and "critical" still make perfect sense when you're talking about just importance instead of importance+time-sensitivity. And two, having more promise boxes to fill means you'll spend more time thinking about it or working on it, which has the emergent effect of it feeling more important. If the goal is important, you'll put more attention into completing it, right?