Honestly, I've browsed through a fair bit of Sarv x Ruv fic, and the most obvious thing is how many writers lean into the 'opposites attract' angle, but not in a simple way. It's less 'sunshine and grumpy' and more like... celestial order versus chaotic freedom. You'll see tons of stories where Ruv's raw, unfiltered expression through music becomes a kind of painful, beautiful truth that Sarvente, bound by her divine duties, secretly craves. The trope of her finding solace in his noise, something supposedly blasphemous, gets repeated a lot.
A less discussed but really common thread is the 'unspoken understanding' trope. They rarely have long, sappy conversations. Communication happens through shared glances during performances, or Ruv playing a tune that somehow answers a prayer she hasn't voiced. There's also a frequent undercurrent of 'forbidden love' framed not by societal rules, but by their very natures—an angel and a demon-adjacent figure, a hymn and a scream. It creates this constant tension between devotion to their purpose and devotion to each other.