To make the cut off corners on the first one, is there a more efficient way ? I used the knife tool and cut a triangle and then disolved the faces individually.

Select the vertices of the corners, press CTRL +B to access the Bevel tool. Press V for Vertex only mode.

I used the knife tool as well, thanks for the answer. I think I need to experiment more with the bevel and inset tools, it didn't even occur to me to use the bevel tool with vertices.