As a reader who's seen this pairing bubble up, I tend to focus on the 'why' that makes it click for some. Kaveh's whole deal is this passionate, slightly self-destructive artist burdened by debt and ideals, while Lumine is the calm, steadfast traveler who's literally seen worlds. The contrast is obvious, but the fics I enjoy dig past that.
Instead of just making Lumine his financial savior or emotional rock in a generic way, the better ones use her outsider perspective. She doesn't care about Sumeru's academic politics or his reputation; she judges him on actions alone, which cuts through his insecurities. He, in turn, isn't intimidated by her power—he's fascinated by it as an artist, wanting to capture it, which gives her a role beyond 'warrior'. It shifts from a rescuer dynamic to two people offering each other a completely different lens on their problems.
Some writers lean too hard on the 'Lumine fixes everything' trope, which flattens both characters. But when it's balanced, you get this interesting study of how someone who carries the weight of a nation (figuratively) interacts with someone who carries the weight of worlds. It's less about romance, sometimes, and more about mutual, quiet understanding from opposite ends of a spectrum.