Since the idea with the game is that people should explore it's world and not think too much about if they have found "everything"
I think you can't stop people from doing this. People can't even stop themselves. I'm my own example: I loved the completionist appeal in Kometen: I wanted to find all planets. This didn't cause any stress for me. Nor did it prevent me from exploring other things and being amused by what I saw on the way.
I don't care much about achievements. Even if I'm a mild completionist in games, I would never look at the list of achievements and feel like I have to get more. I do like achievements as extra pieces of content, when they are smart and well written. I don't care how they relate to my play activity.
Achievements simply open up a game to one or more new play styles, with minor development effort. But you need to take care to implement them tastefully so as not hurt some of the play styles established by the actual game design.