@DouKana684
I would prefer "danna-sama", especially if she changes the way she addresses him later, when they become more intimate and maybe his dominant role in the relationship gets a bit mitigated.
She might change the way she calls him to "anata" or "danna" or she might even start using his first name with no honorific.
With "darling", such a change might be hard to convey... imagine going from "darling" to using his name, it seems colder and loveless in English, but it usually express stronger bonds in Japanese.
I haven't read the novel, so I don't know if it happens though...
Just don't keep "husband-sama"... even darling would be better 😋