Yeah, published adventures are at best character-neutral, and a skilled GM can sometimes frame them in, in ways that might matter (though that's a matter of GM skill, more than anything about the adventures). I think it's maybe arguable that some of the longer published adventures/adventure paths can be about the PCs, if the players bought into the thing at (or more likely before) chargen--though I think less about the adventure/s than about the players' attitude/s.I don't think any rpg material has ever promised this in the 50 years of the hobby. But plenty of materials have bits and pieces that can be used for your campaign. I would look towards things that are geared towards sandbox or even improvisational play.
For my part, I prep situations, and if there are dynamic NPCs present, their responses towards things the PCs might do. And a lot of it can be done walking.
Your approach to prep doesn't sound all that different from mine, though I tend to focus less in prep on possible NPC actions than you seem to (which is a difference, not a right/wrong thing).