If you’re using Megascans trees, that’s probably part of the problem. They’re really made for accuracy, rather than performance. Try to have only a few different foliage types, and work with them, rather than using lots of different things.
This scene uses one rock and one tree, you can do it
Nanite won’t help really. Also, lightmap density does nothing, unless you’re building static light.
You could use much more low poly trees, that still look ok. There’s a good reason why you see a lot of rocks in games, they are much easier to draw 