In the past when changing displays I have just changed the resolution in the json of the .touchgfx file for my project in order to be able to retain screens and just go back in and resize things. It's a little annoying cause you have to find/replace...

It looks like the width in the LTDC config in CubeMX is set for 400 pixels. Should it be 800 for that LCD or am I missing something? I would also double check the resolution is set correctly in TouchGFX and the framebuffer is setup for landscape.