Jin was a state of the Jurchens, Turkic nomads who held the territory of northern China in the 12-13 centuries. Qing was a state of Manchu nomads. Tang was a state of Turkic nomads. By the way, it is precisely the Tang era that is considered the highest flourish of Chinese culture.
An Empire is not always a monarchy (USA, China, South Korea). An Empire is specifically the consciousness of oneself as the center of the world, and others as the Barbarian periphery.
Gumilev — I have read all of his monographs. China is best described in "The Huns" and "The Huns in China," as well as in "The Ancient Turks." These are early works; nothing is mentioned there yet about passionarity (except for "The Huns in China"). And regarding passionarity — one simply needs to read carefully (no offense). There is no mysticism there.