According to the lore of Derketo, the goddess herself is highly revered as well by the peoples of Stygia, Shem and Kush.
If so why there isn’t a temple dedicated for her in the vicinity of Sepermeru? 
Do remember that Sepermeru consists majorly first of the peoples of Stygia, Shem and Kush. (Mind you as well that the worshiping of Set can go in tandem with the worshiping of Derketo)
I suggest that we could have a Palace of Pleasure as placed in a spot of Sepermeru, in opposite direction of the current Set temple (i.e : if Set’s temple is located north, then Derketo’s temple is located south)
Within the Palace of Pleasure, maybe we can place a Derketo priest/priestess kneeling down, while there’ll be two others beside it doing exotic dances.
If you think that’s still considered widely not available, perhaps then we could include them spawning occasionally in exile camps too (maybe goes as well for other religions’ priests/priestesses)
As for the priests and priestesses of Derketo in the vicinity of the Pagoda of Boundless Lusts, I only have this to say to them: No man is an island, you can’t spread Derketo’s influence while being trapped on an island. 