If Kazuya becomes a movie producer/director, being in a relationship with Chizuru will only help his career, but it will be a major detriment to her career. The film industry is tough to break into, Kazuya having exclusive rights to Chizuru will be a major asset, but there is no way that Chizuru will be able to break into the main stream entertainment without going to other producers, which would only weaken Kazuya's position.
Further more, and this is an unfortunate truth, the entertainment industry is all about visuals, and sex sells big time. As talented and beautiful as Chizuru is, there are hundreds of thousands more just like her, with just as much talent, and just as much will power to succeed. Chizuru needs every advantage she can get, and her having a boyfriend/husband/lover will only hurt her chances at getting a role where another girl could just as easily get that spot, all because the other girl can be marketed as "single" and attract a wider male audience. With Kazuya by her side as a boyfriend/husband/lover, she will never attain the popularity her grandmother had, despite their best efforts. So Chizuru will never be fully fulfilled, compounding this unfulfillment, Chizuru will be unable to have children until much later in her life, and by that point (and given Chizuru's personality) they'll have the stereotypical one child. This will leave Kazuya unfulfilled as Kazuya desires a large family.
Chizuru having children will take a dramatic toll on her body, at the very least taking her out of filming for at least a year, and her body may not recover its pre-child form (and definitely won't if she has two children). All together Chizuru and Kazuya are an incompatible couple that will leave each other desiring more out of life. That is part of why Chizuru is not a good choice for Kazuya, or vice versa. A relationship with Ruka is a much more compatible choice, from goals/desires in life, to personalities, those two just click, like they're made for each other. In addition, Ruka acts like a significant force multiplier, back whatever Kazuya desires with 200% of her being, her commitment will help who or whatever Kazuya supports, as Ruka has proven to be flexible, and will grow to fulfill any roll she is assigned.
Sumi comes close to Ruka, and excells in some areas Ruka fulfills, but fails to fulfill Kazuya in all areas, mainly she doesn't seem the kind to have multiple children. Mami will help Kazuya's professional life the most as she has been shown to have no moral reservations when it comes to doing whatever she needs to do to get what she wants. Mami will also give him a large family, but will ultimately leave him very unhappy.