The dungeon you describe works best when the non-heroic party is able to explore it in a non-heroic manner. Generally, this means at least the following:

1. They can explore it at their own pace, fully able to stop / run away any time they are not satisfied with perceived or actual level of danger.
2. They can, at least often enough, guesstimate the level of danger to be low enough to proceed.

The challenge then mostly shifts from direct confrontation with whatever level-appropriate opposition is behind the door you've just kicked in, to:

1. Correct assessment of the surroundings and their inhabitants.
2. Planning, and executing, a reasonably safe way of getting some of stuff that is to be gotten from this dungeon.

Namely, this includes determining if, and when, to leave the rest of the dungeon alone, unplundered, and turn to other dungeons. You generally don't expect a large challenging non-heroic dungeon to be completely cleared, at least not in one go.

I can, and probably will, include relevent examples.

The dungeon you describe works best when the non-heroic party is able to explore it in a non-heroic manner. Generally, this means at least the following:

1. They can explore it at their own pace, fully able to stop / run away any time they are not satisfied with perceived or actual level of danger.
2. They can, at least often enough, guesstimate the level of danger to be low enough to proceed.

The challenge then mostly shifts from direct confrontation with whatever level-appropriate opposition is behind the door you've just kicked in, to:

1. Correct assessment of the surroundings and their inhabitants.
2. Planning, and executing, a reasonably safe way of getting some of stuff that is to be gotten from this dungeon.

Namely, this includes determining if, and when, to leave the rest of the dungeon alone, unplundered, and turn to other dungeons. You generally don't expect a large challenging non-heroic dungeon to be completely cleared, at least not in one go.

As an example of this, take an old abandoned dwarven mine. It includes:

1. A vertical shaft with a rusted lift in questionable condition.
2. A sloped narrow railway with a rusty cart on it, with the brakes that have mostly rusted away.
3. Wooden beams supporting the ceiling that are better not be hit with anything, let alone a derailing cart.
4. A huge storage room with lage crates, large spider webs, and, presumably, large spiders. From the look at them, it is unclear what is in the crates.
5. A smithy and a repair shop, with tools and materials.
6. An office with a locked safe with a log of all things coming in and out of the mine, that, if you can read dwarven and do some thinking, could point out the things that were received from the surface just before the log ends (a lot of canned food) and things that were scheduled for shipping out, but not shipped (a lot of mithril ore, possibly including a listed quantity of nuddets of listed weights).
7. A considerable number of assorted skeletons, most of them dwarven or kobold, doing the mining in a side track. A kobold shaman necromancer leading a small tribe struggling to get enough food, but having a steady supply of the metal.
8. Large metal doors, obviously of dwarwen construction, cold and wet, with a screw that apparently allows to open them. Everything to the other side of the doors is completely flooded.
9. A demonic altar, misuse of which has caused the demise of the dwarves.
10. Levels below, with long-time lack of sufficient ventilation. The party did bring a canary with them, didn't they?

Now, it could be seen from this short description, even without giving any specifics, that there are courses of action that would certainly get the party killed. What is impartant is that most of them could be reasonably expected and, one way or another, dealt with. There are lesser rewards, greater rewards, and greater dangers (or ingenuity requirements) associated with greater rewards, so your players can have as much challenge as they can deal with.