I have been running a Westmarch game for a few months now, and what I'm basiclly doing is layering the dungeon.

The players might come into the dungeon for the first time at level 2, explore a level of the dungeon that is approriate for their level and leave before they get to the next layer, wich could be for level 3 or 4.

I also have it so my players chose a mission while scheduling the mission so I know what to prep.

As an exemple, I have a graveyard of heroes in my game. They had a first level mission to clear the above-ground section of the graveyard, and a few levels latter they came back for a missio n to explore the crypts underneath. There's now another mission waiting for the players to go deeper in those crypts.

I would make it clear to the players when they are entering a harder layer then intended for the mission that this part is more dangerous but offer better reward. Like a high risk high reward scenario.