I have to agree, for the reasons Sc0ut and Windy already brought up, that this doesn't really feel like the end to the first book in a series so much as it feels like a brief lull in action. To me it makes more sense to just think of it as the end of the first story arc of a much longer story (even though there were other arcs, like Sleipnope's arc, within this arc), like how in a lot of manga (and probably other media too idk) one story has a bunch of smaller arcs.
It does make me wonder, though, if Minna would have changed around certain points in her script if she had decided to end the story with this arc. Ex. maybe we have had a longer epilogue (to parallel the longer prologue) that would have given Onni's character better closure, shown the results of the mission, etc.