The cave-in sounds good. The traps with falling rocks are the most obvious. We can't dig a pitfall into the rock or have swinging logs out of nowhere.
As for how to design them? Hmm, not exactly my specialty but Cat was enthusiastic to learn about traps.
We have a ladder so Cat can climb up to the ceiling with a pickaxe. She'll loosen a few rocks and insert a timber as support secured by a wedge. For that we have pitons that can be inserted and hammered in. From the wedge, a rope leads down the wall. After finishing, everything can be blackened by soot from a torch burning with oil. We can build a barricade at the narrowest part of the main entrance from the debris laying around, Maybe even move one of the mine carts there. Debris left and right of the tracks, mine cart at the middle. Rope attached to the cart. When this is fished, a high DEX expendable sacrifice (Alice) can climb up again and carefully further loosen the rocks until they're only supported by the timber. When the enemy tries to push the cart out of the way, the rope pulls out the piton above and the rocks fall.
Downside:
Can't be safely disarmed, once the rocks are loose, they will fall at some point and damage the tracks
I think it's ok if this trap is obvious, it makes them go through the South corridor and into two additional traps. If they spot those, they''ll assume the main path is just blocked off and trigger the major cave-in.
In the South corridor we can use pressure plates on the wooden floor that trigger something when the enemy steps on them. But we should make these traps disarmable. We want to get out of the mine at some point. Here we could use a similar method to make blades from the bandits weapons fall from the ceiling. An alternative would be to use the bugbear commander's axe on a swing at the end of the corridor where I assume the ceiling is higher. Tie axe to a rope. Secure rope on the ceiling with a piton. Secure Axe high on the wall opposite of the passage. Rope leads down to a pressure plate at the end of the corridor. Step on it, the piton securing the axe is pulled out and it swings into the corridor causing havoc.
1) Cave-in at narrowest point of main entrance
2) Falling Blades at middle of South corridor
3) Swinging Greataxe at end of South corridor
