I have a pair of exterior doors in my shop. They are the normal metal "raised panel" steel doors. I have been considering making a new set of doors using an engineered type approach. I have built exterior doors before using solid stock and the typical shaper cutter approach.
So here is my idea for an insulated 2.25" door. 3/4" frame work with stiles and rails. Interior area insulated board. 1/4" exterior ply glued to both sides via vac bag. Then 9/16" cedar T&G on both sides.