I have built an extension with solid walls and 200mm EWI. The windows and doors are fitted within plywood boxes so they cantilever to the mid-insulation position, the windows were no problem but the doors required extra support. When I stood on the outer edge of the door boxes they deflected down. To counter this I doubled the ply to 36mm (screwed and glued) and strapped the inner edge down to the concrete slab. This worked well and made the lower edge very solid.
I am now planning my next phase where I want to do the same thing on the solid wall bungalow but with bi-fold doors about twice the weight as the previous largest door. Do you think the same approach will work without much deflection or do I need to make a stronger box?