Yeah this exactly..I thought it was fine for what it was. It wasn't really a character driven series so the lack of development isn't too big of an issue, in fact forcing the issue between ange and charlotte any further would have felt just that, forced. They were toeing the line between a "caper of the week" spy hi-jinks show, an overarching plot (or at least foreshadowing) with the ultimate end goal of taking walls down, and the character development required to have you care at least a little if they succeed or not. All in all, for an anime, I thought they did pretty well.
Btw Made in Abyss OST is released and of cuz this godly insert song is included >.<
A bit surprised that the singer is actually a japanese from hokkaidou poi