Thanks for the details mate.
We currently just copy the prefab from the original file into our game map and it seems to work so far.
We will add a large building in the next days and see if it drops fps.
If this doesn’t work we will try the export to 3rd party and back stuff.
The collision part is no issue we had that previously while importing c4d objects to Unity (should be doable)
The door mechanics issue is way bigger and I have no idea how to tackle.