Custom Mesh for Game - Edges of my meshs' tris keep appearing on flat surfaces

Hi,

New to using both blender and unity. I made a mesh of a large rock (pictured) in blender as a test mod for a game. The game is stylised as flat low poly. Originally, I used tris for the entire mesh, but I found the edges of the tris appeared as shadows in the game, even on flat surfaces of the object. So I decided to have the face edges where actual edges and creases of my rock will be (it'll be converted to tris for the game anyway, so I figured this wouldn't matter much), however, when I use my mesh in game, the converted tris on my mesh still appear as shadows on my object (pictured). In unity, the mesh has a single, standard shader. Metallic and smoothness right down. Specular highlights and shadows off. I've tried playing around different settings and I cannot figure out what I can do to get rid of the shadows. I don't even know if this is an issue with unity or something unity can fix.

Any advice here would be greatly appreciated. For reference, you can see my mesh has one large flat surface in the first picture, however, in the second picture (taken in game) this surface has shadows where tri edges. It's really not meant to be like this. Thanks for any help!