This there a way making a gameobject recieves GI shadows only(lightmap based direct light shadow)

Guys, I am deading with a large scene and have a fairly large terrain in it. Most of objects cast shadow directly on the terrain. But as that i can not give the terrain a surfficient GI baking light resolution(since even 2048 is not enough for the terrain), shadows on the terrain look expectedly blurry, no supprised here. So i figured couple of solutions to solve this issue, not sure which one would be the best(or posible).

0 multiple terrain objects with different GI resolution scale setting?
1 putting planes beneath object casting shadow,and recieves GI shadows only.
2 precomputed GI solution, turn baking GI off.

I'm leaning towards to the 1, since the project will live on mobile devices(baking GI is more suitable for phones than Precomputed GI?)

But here is the question:
The shadow plane recieves the gi shadow correctly ofcause, but i have not figured out a way to turn the diffuse color off depart from the GI shadow, the "shadow only" on the inspecter is not an option, is there an "easy" solution? if only a custom shader can solve this, could someone give a light on me ?(I know nothing about shader)

Thanks!