That powerup isn't that big. Sure, he's probably the strongest on the ninth floor now, but even so, he's barely anything compared to the beings on the lower floors.
It's more of a necessity than anything
Miyu doing lesbian shit just ruined the story imo
Lost all interest in this manga
(FYI, I don't mind lesbian stuff, I just heavily dislike when a character has sexual relations with multiple people, just something about that strikes a cord with me)
The series has ~200 chapters though the fan translation was dropped. There is an official translation; I am unsure if it is free or not. You may be able to find a continuation on another manga website.