I'd LOVE to RP with you if you're still interested! I have an Uke character who is exactly what you are looking for. I do not have any limits for my RPs so extreme dark stuff like blood, pain, rape, ect... is perfectly fine with me. I'd be more than happy to satisfy what you're craving.
Here's a brief description of my character:
Personality: Confident, defensive, feisty, and knows how to fight and stick up for himself. Loves messing around having sex and is definitely not a virgin. Lol
I'm open for anything or suggestions. I mostly just have basic modern plots similar to what you have already suggested; so anything you are in the mood for let me know. I would love to hear more about your sadistic Seme character or a profile of him!
If you have any questions, feel free to ask. Thank you and I look forward to hearing back from you!