I love the Anal whore Kaeya Headcanons! they are so good! honestly i feel like this whole thing could have gotten started when Kaeya got spicey and got his clit pierced. During the healing Diluc couldnt touch and his hands and dick were swatted away by Kaeya "To avoid infections!"
So Diluc just resorted to fingering his ass and Kaeya liking it WAY more than he wants to admit... Diluc now gets so marvel at his dick pounding deep whilst Kaeya's piercing gleams and sparkles with slick... taunting him at every moment...