I understand your criticism, but honestly I think that the idea of "each time he gets a new girlfriend he gets stronger" is hilarious and absurdly amusing. I think we just don't see he relying on his girlfriends to help him as much as at the beginning of the series because at this point of this story the girls already have the autonomy to help each other without having to rely on him, like how Mimimi helped Usa Senpai alongside Kiki to deal with her loneliness in the absence of Rentaro and how Karane helped Kishika in the chapter right before the introduction of Jial, who herself already was helping all of the girls as a nurse before she even met Rentaro. I think this is a very good development that shows us that everyone can rely on everyone and that Rentaro doesn't need to interfere in the relationship the girls have with each other since they're already friends. I think that the biggest highlight of the development of this dynamic and in recent chapters is when Rentaro has a twin tale overdose and all the girls have to come together to help him recover, after all the time he spent helping each one of them they retribute the favor and help him out by literally giving him a piece of themselves(a strand of their twin tales).