>one artist
>visual novel
Yeah, see, there's your problem. Go look up how many drawfags King of Dragon Pass had.
You're making some ren'py-tier shit, that means the ratio of coding is minimal to the art, meaning that drawfag is the crucial component of the actual development process, not you. And he's probably overwhelmed. And you have no control over him besides not paying. And he knows it.
He's most likely a relative noob and operating waaay outside of his comfort zone, having to solve problems he didn't know he had. Perspective, anatomy, landscape etc. visual arts are a terrible thing, you're always at risk of running into something, that'll ass rape the quality of your work. You sit there and try to imagine how some character's face should look from an upward angle, while smiling, then draw it 10 times, then go into a stupor, snap out of it, then pick number nine and touch it up for 3 hours. And it still looks not only bad, but noticeably worse, than your usual stuff. You realize you lack the mental workshop to deal with this, but you don't have time to crack open a 300 page book. So you go on to the next thing, which also has a 10% chance of stumping the fuck out of you and sucking the fun out of all this.
>and I can't just change him for someone else since he has already done a lot of the work and I would prefer to keep the consistency.
You boned yourself there, didn't you? If he knows you have no ability to exert pressure and are, in fact, the one dependent on him he'll take extra time to:
-go limp and wallow in his own shit over how much of a not-Sargent/Frazetta/etc. he is,
-try to up the quality of his work beyond reasonable effort per piece (in the case of someone who can do a lot in one go this behavior's the rarest),
-just fuck around and find excuses not to work.
My guess is you take two unaffiliated yet similar artists, make them work out a style you all can agree upon and they can work with and ==then== hire them.