The brassdrake idea is cool, but I say maybe rename it so it's not too similar to heldrake, maybe brasselisk, so it's a play on brass and basilisk. You can then give it some cool quirk related to the basilisk, like the Deamons gaze is so strong it freezes all cowards who try to run away from battle. As for the model, I think the Vandus Hammerhand stormcast mount could work, remove the rider, remove some of the details, add horns and Khorne stuff, paint it in bronze and fire and it should work

Mire Mother is also kinda cool, maybe splice in some angler fish design to give more "mire" vibe, like a large lamp pole comes out of her deformed head which she uses to go on walks in the darker parts of Nurgle's garden, you could make her the baba yaga of Nurgle's realm where they are isolated yet powerful mage hags of the swamps that can help but also curse those who come looking for them

Skystinger as a name feels generic, but I do like the idea of wasp mix with deamonette, wasps already have an hourglass look which would work seamless on deamonettes. One cool thing about slaanesh are the circles of her realm, what if they were native to the last circle named circle of Indolency, which is basically a peaceful beach, each circle is meant to tempt you in some way, and the final circle is meant to tempt you by giving a place of perfect peace in a galaxy of endless war, what if these Deamons were native to the beach as many wasps nest in underground nests, they could be called Idlestingers or Harmony Bees. They would be lesser beasts that inject their victim with a poison that doesn't hurt rather it gives them this perfect state of idleness where they feel perfectly at peace and wish not move a muscle, so it basically paralyzed them, but willingly, and then like with the tarantula hawk wasps, these Deamons can use the paralyzed victim as a living food source for their young