Sometimes SH is compared to Settlers, but I usually compare it to Dungeon Keeper. There was a very useful feature in DK: if you locked a door, you can instantly see that it’s locked.
- It had a hovering icon of the key above it
- It had chains and a lock visible on its model.
The first was made for 3rd-person “manager” view, the second for 1st-person “inhabitant” view (but also helpful when watching from above).
Since there are now lockable doors made by the Engineer, I suggest to do the same for them. Adding visual elements (key/lock icon and different model outlook) will help planning the routes of workers without clicking doors one-by-one.