I'm not a GPU programmer, but I became interested to dig out what is the state of art of culling, and found out Arseny Kapoulkine [1] has implemented those in MIT licensed C++ code base:
Cluster cone culling, frustum culling, Automatic occlusion culling, triangle culling, Meshlet occlusion culling, Optimizing culling. On paper sounds impressive, and maybe Colossal Order could learn from those.
https://github.com/zeux/niagara