I went digging a little in this series and I found several things:
1- I think we all know this an adapatation, the curious thing is that the Light Novel only has 2 volumes released in 2020 and that is it, it probably wasn't continued because of low sales and I can't really blame the readers lol
2- All the material used in the manga comes from the web novel that still gets updated, latest chapter for the web novel was released today in fact, but the manga deviates a little from the web novel.
Here is some info that I got from the web novel so far:
The main wives are as follows and that we all know:
1- Luna (Main wife).
2- Valerie.
3- Astrid.
4- Hilde.
Why Luna the main wife? The theory is that since she was the first heroine and the one that manages the household, she is the main wife.
Regarding Angelina, Mina's mom and Luna's mom, there is no mention in the web novel of them being confirmed romantic interests, however; the manga does not follow the source material 100% so we shall see what happens later on.
3- Regarding Luna's subtle hint in the last chapter, there is 0 confirmation that she is indeed pregnant in the web novel, the explanation is that since Luna is the main wife, Noir and Naomi tend to overprotect her.
4- There is no mention of Noir being a he or she in the manga but in the web novel it is mentioned as a she, quite interesting.
5- There is a whole theory of why all 4 of them are the wives that involves Reda's healing power but it is so convoluted that I won't try to explain it, just know that it involves Reda's power touching people's soul and leaving a warm and fuzzy feeling.