There are several problems that need to be dealt with first. 1 is the height of your house, the height of the door and the height of the beam on the door. This is what you must know when you change the arch. 2 Where is the external corner wall you mentioned? Top？ Or the window? Personally, I think you should be talking about the external angle of the wall, that is, the foreman's trouble is to knock out the external angle according to the size to make a fillet, and then bend it into a non-circular position with the joinery board at that position, and then it can be treated directly like a wall. Personal experience suggests that it is better to add gypsum board to the surface of blockboard (just saying it is better, not saying it is bad)