I think that is acceptable, not exactly good of bad, but acceptable.
It's would be better for peace since she is the demon king successor while the actual demon king looks like Buddha encarnation itself after the brainwashing.
But I'm only saying considering that will stop on her.
In a terms of future, having each races being mixed blood with each other, being a family with each other starting from the head, would paviment peace along the eras if well done.
I agree with your sentiment, but it depends on how is done, I don't like those sort of harems where girls are only explored at a surface level and the author feels a need to re-assure the readers that new girls will appear, even in those Isekai where the protagonist actually gets game, the harem can become overly stuffed and I think the nice thing this one is doing, is skipping various steps we typically see:
Typically the fight against the demon would have taken more time, the hero realizing the feelings of everyone, them kissing, the wedding proposal, living together, going to THE ACADEMY(tm), getting a noble's title [insert obligatory rival at the academy].....this might be a monthly release but is going comparatively quickly and I don't want this pace to mean that things have to start and end too quickly and treat their daily life as a simple story beat to get over quickly......while I don't ask for the story to become a SoL, I'd like that element to always remain in the background while the story progresses, because too many Romcoms fail to understand the way people change once they are together, but this is also due to Japan's stupid idea of "oh we can't be seen be romantic together", somehow magic is fine but romance is too alien.