I feel like there’s more to her character than that

That's actually the reason why W falls flat as a character, there's more to her but the story doesn't know how to get it done in a way that feels natural to her character progression. It's perfectly fine that she has depth, she's one of the most plot important characters, so she will have to be more fleshed out than being a mere crazy terrorist in one way or another.

And you know what I think to be the biggest culprit in this? Darknights Memoir.

It's an event that's meant to be centered around W's character as opposed to the ongoing civil war and the fragmented state of Kazdel, so here, we're supposed to get to know her better than how she's presented in the main story. She starts out as an apathetic, unapologetic mercenary with no qualms for collateral damage. One day she met Theresa, the current King of Kazdel, whose idealism provides a nice contrast to W's cynicism. Just like that, she became Theresa's loyal puppy, and whatever transition she underwent as Babel's mercenary and a follower of Theresa happened off-screen. What we got instead is her development needs to be told in the narrative, since they've skimped on showing it to us, completely disregarding whatever her character once was before DM. It comes across as thoughtless and rushed, when whatever depth she presents afterwards is chalked to a mere blind worship that is frankly uncomfortable to watch.

So you end up with a character that's supposed to be compelling in motivation and personal conviction, but it turns out said depth isn't really much more than a fancy cardboard cutout created by the writer to give an illusion of depth.

Heck, I find the bonus scene with the kids to be far more telling about W's character than the entire drivel of DM. It's simple, to the point and builds upon the W we've come to know, as her method of helping people in desperate situation is very much emblematic of her harsh background and hidden, but twisted sense of empathy. We can tell W isn't just being compassionate, she's also project herself in this situation. She may not be a complete lunatic, but she's not a great person no matter what lofty ideal she follows, and the plot puts no effort in hiding that fact in this instance.