The dude has been tying up girls for long enough, he needs to get serious and make an actual harem. A few girls, probably Sarah and Scarlet, need to start talking to each other about being wives, not FWB. It would work better if not everyone was on board. A little tension and insecurity to make it interesting.
Fang and Yuki should start calling him Master once they realize which way the wind is blowing. And the older women should start to become more submissive in their behavior, realizing they can't be wives, but secretly hoping to be concubines. I'd especially like it if Nadia became a lot more submissive At least one girl from the Organization should volunteer to be his slave, not feeling worthy of the consideration. Maybe it starts with Rue, but then someone else gets interested in the idea, like Lyra.
Zero doing petplay seems better to me than the short episode with Sarah and Jeanne. She should be enjoying it, with the leash and the plug, and trying out a lot of new locations and behaviors. I don't like the Lethe/Zero matchup and I think Lethe would be better paired with Daisy, to learn some nurturing instead. Other appealing possibilities are Hazel+Helena for pragmatic thinkers, or Jeanne+Leah, for thikkness.