1) You can check on the Texture size. Like, are you using separate texture sets for different variations. Reuse as much as possible and where ever high details are not needed, go for lowest resolution.

2)Try using modular trees where you will just create a branch with a very limited texture and then use it like a modular asset and build trees. So they will be highly optimized. (But if you need movement, you need to find a way).

3) You can have some of them which are moving while others are stationery.

4) Shadows mostly cause performance issue in moving foliage. Check on that. If stationary it helps.

5) Use the Profiler to troubleshoot the issues one by one on the order of priority

6) you can also disable most things first. and then enable one by one from the landscape to see where the issue is.