Considering the purpose of a wife here, Nonna is the logical choice. He might not have thought that, but circumstances developed that way. I suspect everyone is second in his eyes, except the vampire who is first in very way.
And she will make that very clear in the ending... I personally really dislike the way that was done in the novel.
I think he should have come back as a Vamp a few days later, beat up and exiled his idiot son and retake the throne... Or at least get his harem and Vampire them too, then go live in a community somewhere.
I hope they change it in the Manga.
From a logical point of View, Nonna is the best choice as an Aristocrat's Wife, because of her Knowledge on medieval noble society...
But i somehow just do not like her, and only because of that god awful personality.
Whenever Ägir is off to war, Nonna will spend the time bankrupting him by buying useless amenities no one ever needed. Ägir could easily put a stop to this by putting his foot down and prohibiting it, or giving Adolf permission to shoot her attempts down, but he never does, and so she seemingly only gets worse and worse. Also horribly entitled to the end. Just full on Karen.