All of the Dax hosts were still Trill, and it was issues with the sixth host, Joran, that led to Sisko learning about the lie being told about just how many Trill could actually be Joined.

The fact that Jadzia was preceeded by Curzon was immaterial to the symbionts as the whole point of Joining was to maximise experiencing life. Which was also the reason resuming relationships was so taboo as it didn't add anything. The one time this gets explored in depth was the lesbian relationship and kiss the show had with Jadzia, while Ezri hooking up with Worf felt more like bad writing and drama for the sake of drama given how many episodes it lasted. Dax and Khan was a one off for the contained episode and while debatable how much it gets used as the tip of the wedge, didn't hang around longer than it should.

The show could have simply added a non binary Trill if that was the entire intention but for some stupid reason they decided to go that extra step and make the host human as well!

The whole episode where a rejected candidate steals the Dax symbiont shows just how significant the whole Joining process is to some Trill and modern writers thought it a good idea to canonise "permanent" non Trill hosts?

Idiots. Actual idiots.