Shizuka's not a power-bottom. She certainly wasn't with Kaoru because it's pretty evident that Kaoru prefers to corrupt good girls. That's why she cooled on Hibiki so quickly. See the cotton candy scene at the festival.
Look at Mio and Shizuka's early interactions. Even the moves Shizuka makes that look aggressive are invitations for Mio to take the lead. I think Shizuka was playing at the seductive senpai role to spite Kaoru, but she couldn't help revealing her true nature. She seizes the role she really wants at Valentine's, giving Mio chocolate and assigning her White Day responsibilities. Their first kiss is her inviting Mio to kiss her. She remains seated while Mio stands over her. Shizuka doesn't want control. She only steers Mio toward topping her. She's all bottom. (as mentioned, in a relationship power dynamic sense.) I don't think what Mio exhibits early on is Top energy as much as the single-minded possessiveness Shizuka couldn't get from Kaoru. Shizuka 100% wants to be someone's sole possession, but the most she would ever be to Kaoru is the centerpiece of her menagerie.
https://dynasty-scans.com/chapters/virgins_empire_ch126#6
and then Mio, without realizing it, auditions to be her possessor. Shizuka's eyes on the next page are her realizing that Mio is actually everything she wants. The same eyes as when Kaoru realized she was losing her grip on Shizuka. The same as Kaoru falling in love with Mask at a glance. They're significant shifts in the love triangle. The die is cast in Ch. 126, and what follows is the necessary shedding of a toxic relationship to make room for the new one. Kaoru is a fun character, and I think she could have an amazing redemption arc with Mask, but she still knowingly manipulates Shizuka. It's only this day in the story that Mio begins to understand she has to take the lead. She sees Shizuka's weakness and realizes it's up to her to pull her out from under Kaoru completely. She's always been stronger, braver, and more honest than Shizuka. She only lacked confidence while Shizuka's confidence was only ever a facade. Losing your virginity can be a big confidence boost, so I'm really curious about what happens next. Mio's not going full Yuu-sama anytime soon, but Shizuka's definitely down for the spanking.
