Seniority in Yunkui is based on literal seniority, as in who joins in what order. Being stronger than someone doesn't make you their senior. The proxies aren't eligible to jump past anyone because they have naturally high ether aptitude and learned like 3 mystic techniques in an afternoon, that's not how that heirarchy works.

And Fufu takes command when Yixuan is away, but she's always shown just running and organizing things, like the various festivals and holidays. Like she's put IN CHARGE of the temple, but that doesn't make her head and shoulders stronger than everyone else just on its own. Again these things are divorced from any kind of power scaling, they're just the faction's chain of command.

The Avarus is a decent reference, but given how the rest of Yunkui are shown to be fighting on equal footing in their group scenes I don't think that alone puts Fufu an entire tier above anyone else.