I see.
I think you missed the point of this entire game, and of Kohaku. What's notable in the story is that Kohaku has some background that makes her rather important to the story, and also that the whole reason the story is taking place in the first place is because of the cursed rings that Kohaku and Jimmy(the protag) are wearing. Jimmy canoncially, couldn't find any friends whatsoever to party with, which lead to him being a solo adventurer up until Kohaku showed up and they became a couple.
Storywise, this game is about a couple going on a journey to save the world and whatnot. Combatwise, you have a front-line tank/damage dealer in the MC, and the support/healer in Kohaku. Potion Maker and Yarizo are barely relevant in combat, because they don't need to be, the two of you already cover every niche. In fact, taking Yarizo actively detriments you because at high addiction levels, he interrupts Kohaku's casting.
Kohaku barely tolerates Potion Maker as is, and goes out and kills any woman you have sex with besides her. There's no way a harem ending would slide unless Kohaku dies in the course of the story.
I'll admit that Technobrake is getting sidetracked with other girls, like Iris, Potion Maker, and the various other girls in this game and their designs, but the focus is Kohaku. She is the titular Nymphomania Priestess. No one else can hold a candle to her in number of scenes.
TLDR: Sure, more girls and party members could be added, but why tho