Don't know if people already suggested that, BUT:

1) Reuse DistanceCulling asset from any of the big maps like Gorodok or Yehorivka on yours - the fact that every object is being drawn across the map is killing the FPS.
2) Set maximum culling distance for trees and all the vegitation except grass, because you already done that. Trees are expensive as well as any other mesh, and when they are being drawn across the map - same result as 1)
3) Water needs a different shader. Consult Modding Discord and maybe ask Zeno, because his next official map Skorpo is going to have lots of water and their FPS and looks were ♥♥♥♥ as well.

I wish you luck my dude. The map has potential. Feel free to ask me whatever on Discord - you can find me through Official Modding Discord, my name is the same as this one.

I've got some experience working on maps, so I believe I can ask some questions for you.

Keep up the good work dude!