Aside from building the frame yourself and just using the slabs. Aside from counter jamb extensions, is the fiberglass not solid? I shortened 7 steel doors with at least 3” removed only to look factory at the end. If the threshold needs modification there is the hard part. One could replace it and build the jambs no?
I would do oversized trim and notch it around the protruding jamb, kinda like brick or stucco molding. Caulk n paint make it what it ain’t