How do you get lost of trees to render at speed in Unity?

I've just tried to tree-ify a large terrain 8x8 1km square tiles with standard trees conifers and pines and it looks great but has very strange benchmarks?

< 20 fps
< 20% CPU
< 20% GPU

Deep Profiling seems to indicate that the PerpareSceneCullingParameters is part of the bottleneck with > 60ms per frame in this method.

So how do you get large woodlands to work with Unity?
Or what are the limits to the current foliage system and how to work within or around them?

PS: I've also thought that maybe DOTS could be used to solve this problem and raised the idea over there https://forum.unity.com/threads/could-dots-fix-the-unity-terrain-tree-performance-problem.931554/

Is this thread the most in depth study into the issue? https://forum.unity.com/threads/case-1227083-tree-nature-soft-occlusion-shader-in-urp.825825