Hey, guys. I found a trick to simplify generating grid on uneven terrains. I though I'd share in case anybody finds it useful.
What I do is use free unity asset Pro-Builder to create a grid painting helper mesh.
I start with a Pro-Builder cube with size same as my grid. In my case 1.5 units. (You can set parameters by clicking + sign in Pro-Builder window) Then I set the extrude parameter to same size, so it extrudes by 1.5 units. I kept extruding and moving edges up or down, until I got it aligned with terrain shape I needed.
After this I used my new mesh as base for generating the grid. Because it's hard edged surface and same polygon shape as your grid it creates pretty good Grid shape for you to start with. Be sure to match your grid position to Grid Helper Mesh's position. It's easiest if you generate grid from one of the corners and use offset feature to perfectly align grid with Grid helper mesh.
Not an ideal solution, but much easier than messing with Unity's terrain smoothing.
If you are willing to write some custom solutions or buy paid assets you can further simplify things by "Stamping" your grid helper mesh into terrain. personally I use Terrain Former Asset's "Mould" feature with "Top-Down" mode activated. This allows me to match terrain shape to grid helper mesh with brushes. https://www.terrainformer.com/videos/shrink-wrap.mp4