There's this flute melody in the Saiunkoku Monogatari main theme that sounds like it belongs in an ancient Chinese court drama, which is exactly the point. The whole OST leans into traditional instrumentation mixed with mid-2000s orchestral anime scoring.
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Hajimari no Kaze sets this hopeful tone right away, then Shuurei's character themes shift between gentle piano passages and strings that actually hit harder than you'd expect from a political romance series. Tsuchita Usuba Ashi has two versions here, one ethnic and one acoustic guitar, and the guitar arrangement is honestly the standout.
That quiet ache to it. If you're into Twelve Kingdoms or Fushigi Yuugi soundtracks, this one belongs on the same shelf.