Seeing the sadly large number of unanswered posts on these kind of topics, here is the answer to this after nearly a week of stress.
There were only 4 issues that broke the collision:
- Unreal doesn’t like Cinema 4D’s fbx export (normal smoothing warning), so I run it through Blender’s export set with face smoothing too
- “Complex as simple” collision can only be used on a mesh that you expect to stay in place (documented who knows where)
- You are expected to export your own collision using convex shapes with the “UCX_” prefix to trick Unreal into having pseudo concave collision on the simple collision channel
- You need to click “import collision from source” button in geometry collision settings which imports the simple collision which is just as complex now as the actual complex collision
The Destruction documentations skip these steps in their “best practices” sections.