I think the ideal line, if I were the author, is for Ryouta to agree to be a donor for Suzuka.
And that's it. He denounces his sperm donor, he denounces his mother, tells them both to never involve themselves in his life again, and that he will never see Suzuka again. Being a donor is a one-time thing, and this is single procedure is all he will agree to.
The dad is a shitbag. I don't care about Nitta's speech about how he "protects what he wants to protect even if he has to be the bad guy", that's just him being selfish. He already destroyed Ryouta's family, on the basis of not wanting children—only to ask his unwanted child to save his "wanted" one for whom he destroyed the dreams of other kids.
A man like that is owed nothing, and Suzuka should be saved only because she's fully blameless in all of this.
But I hope Ryouta reads both him and the mother the riot act and shames them into perpetuity, holding the fact he saved the child they wanted over their heads.
Honestly I'd go a step further and have Ryouta spread the truth around publicly and shame the two of them on a macro level, given how Japan operates on social image. But that's a scale of melodrama that runs counter to the tone of this series, so.