Just trying to help. To answer your question on how we place our plants:
I worked on scenes were we had something like 30.000+ trees in it. That was a few years ago. Running with a 1080 probably on maybe 20-30 fps. We placed them with the foliage tool and set them to be dynamic and only cast dynamic shadows. With static trees we would get the out of memory crashes. It was mostly fully dynamic lighting with no interior but lots of exterior buildings and meshes.
I’m currently working on a ray traced scene. A large house with 4 bath rooms, 2 stories one under ground. I modelled what can be seen from the surrounding. I placed most speed trees by hand to enable ‘evaluate world position’ so they have proper self shadowing when moving by wind. The ones further away are placed as foliage or as groups of instances. Maybe about 150 trees plus other smaller plants. LM resolution is 128. I have foliage with 18.000 grass meshes (resolution 16) on the lawn which are set to static and cast dynamic and static shadows. I have also grass on the terrain but it spawns dynamically around the player with the landscape grass tool so hard to say how many meshes. They are set to dynamic otherwise it would blow out the lightmap size and probably crash. I’m using a laptop with a 2070. Production lighting might take about 3-4 hours on the laptop. Preview takes less than an hour.
Building lighting on a swarm network with a few computers took 15 minutes in production mode apparently but I don’t have access to that locally.