It is very fun if you want to be sure that you aren't missing anything the game has to offer.

You've hit upon the crux of the issue, in my opinion. FromSoftware games in general are built on exploration and discovery, finding crazy cool stuff in some dark corner of the game is a big part of the experience. However, for discovery to be properly rewarding you have to allow for the possibility that the player will just miss the stuff you've hidden. Indeed, in a blind playthrough of Dark Souls you're likely to stumble upon a bunch of different secrets and still miss 50% or more of them.

That's gonna be excruciating if you insist on "100% completing" the game. It kind of goes back to older days of gaming when there was no internet and no guides, and you just played the game and were happy when you saw the credits, and had no idea you even missed anything. I feel like modern games with their map markers for everything and completion percentages visible have kind of changed the way many people approach games.

Not to say there's anything wrong with using a guide, play the game how you like. And there is definitely an argument that if you bought the whole game, you'd like to experience the whole game.