Yeah, performance is definitely a concern here 
In my case I’m dealing with thousands of underground elements, so this approach probably won’t scale well enough. Still, thanks for the effort and the suggestions 
if I run out of other options I might give it a try.
For now I’m trying to keep things as lightweight as possible: using primitives wherever I can, disabling depthTest in the renderState, and doing outlines with an inverted hull. It has some flaws, it does not work well enough.. but it works.
I also tried implementing the same idea in Three.js with 3d-tiles-renderer. There it was easier to experiment because I could manually call clearDepth() inside the frame loop and render specific things in a controlled order. But in this case I’m loosing all the good stuff that comes with Cesium… From what I understand, Cesium doesn’t really expose similar control over render targets or render order, unless I’m missing something?