My players used to dig up from up above to the dungeon to avoid dangerous parts of it, on which basis am I in the right to deny them the ability to do that? Hell I would've done it myself. So I don't see a big difference between a clever solution and totally fucking up my plans, which ofc is an achievement in itself for a typical player

If they are capable of doing that and choose to do it, let them. I can imagine all sorts of realistic potential issues to taking that approach (with certain dungeons, at least), but if they can pull it off, whey shouldn't they?

The idea is that you come up with the dungeon, put it somewhere, then set it aside and create something else. When they encounter it (if they do), it could end up being a short excursion or a long one, depending on what they choose to do.

People tend to use the term "sandbox" in a couple of different ways. Some people set up a map and then rely on randomized encounters. Some build a world, populate it, and then go from there (my approach). Some mix the two. In all "sandbox" approaches, though, the idea is that the GM isn't making specific plans that the players can derail.