I just checked the shell and inside door surfaces with a long bevel guage: my doors are sprung out on the forward edge! So I’m with Fritz: keep adding lumber till it all fits: first attempt will be that 3/4″ x 1″ trailer bunk stiffener up against the forward window sill and lapping 5 1/2″ onto the existing door bottom stiffener almost to the bottom of the door. I’ll do the same cut in the existing stiffener that you did except perhaps deeper into the fillet. I think you will find that the glass over your trial cut won’t be enough , probably you’ll have to add lumber. Feel free to disagree/correct me.
My camper won’t leak and is good enough to take on the road, but I would like to get my door fit better than it is so far. Gonna have to sand some varnish off and add lumber.