After volume 6, the story moves from being about the struggles of being teleported into a hostile environment and then traveling across several continents in order to return to the main character's homeland to the main character going to a magic school to fix his erectile dysfunction. Seriously?
While Sylphy is definitely grown a bit immediately after volumes 1-6 ends, not much else happens after she gets pregnant.
In volumes 1-6 Eris is developed a ton, and this period of time happens to be one of the best developmental phases for a side character that I have ever encountered in a light novel. We see her transform from a bratty noble princess into a warrior who has to overcome her inner emotional struggles as they travel the world. Then she is almost entirely forgotten for ten volumes till the author decided to bring back her and Roxy as yes-men (or I guess women in this case) wives who are completely shallow with almost no function other than to have s*x with Rudeus as a plot device. Their personalities have been reduced to nothing but the most prominent behaviors that they exhibited in the developmental volumes.
In short, his wives are transformed from unique characters to generic female archetypes commonly found in other novels of the same genre.
Eris doesn't go away for 10 volumes.
To further expand on this: Rudeus should not have married Sylphy and Roxy. And he wouldn't have if the traveling arc was continued. They were past characters that he only somewhat interacted with for an entire volume each. He grew up with Eris and encountered many hardships with her for four lengthy volumes. The entire situation where Eris just gets up and leaves under the premise of "needing to get stronger" seems completely forced and unnecessary for the progression of the story.