Optimizing and generating terrain on mobile

So I’ve got my mobile game in which a player is controlling a flying plane over a 1000x1000 terrain. Since I’m flying over a terrain I need it to be huge, so the skybox won’t be as visible. I don’t really care that much about the quality, it is really simple as of now, there is a trigger when player hits it, but still - drawing the terrain gets my draw calls to over a 100 easily.

While my SGS 6 keeps a steady 60 fps, Motorola G from 2014, which is on Android 6.0, gets down to 14-18 fps, which is unplayable. I tinkered with lighting, quality of images, removing unnecessary assets and other stuff, but it looks to me like terrain is the thing that consumes the most processing power.

How do you guys deal with massive terrains in such situations on mobile? Using meshes somehow? Or do you stick with Unity’s terrain?

I have been on the same path for 3-4 months and finally find out how to deal with massive terrains.
My terrain suppose to be at least 4k X 4k but i got low fps when i use unity terrain. I have planes, tanks, choppers and tps character in the sceene and i needed an huge open map. My aproach was totally wrong. I tried to make one single terrain holding the whole area with lots of heights. Even 500x500 flat terrain cause low fps. When i increased the tailing and decreased the resolution, got rid of shadows, real time lightining and updates from scripts i couldnt make it more than 40fps. Occ culling didnt help much. The main problem was the terrain itself. Texture quality doesnt matter. The terrain is so dense for mobiles.
I used same scene with mesh which has 3 lods and fps jumped to 60. Mesh was 5000x5000 and very mountanious.
So what i figured out is; you can combine small terrains with meshes and disable them asap when you are done or use chunked meshes with lods. Either way your fps goes up. I suggest you to draw your terrain with unity terrain, add trees and plans, convert it to mesh,when you sre done use mega splat or a atlas asset and bake the meshes and textures. Slice them as many as possible and use lod. This is the most efficent solution. Disable and enable when the mesh is in view. Hope it helps.