Product & technique recommendation to trim this door
Purchase a 1" thick by 3 1/2" wide PVC trim to go around the 3 sides of the door. Ripping to width were it fit to a proper set back from the edge of the jamb in all areas below the masonry cap of the walls. In essence, trimming in out in a typical fashion the way interior doors are trimmed.

A thinner trim would work but the thicker trim allows the back side that meets the rough wall to be cut out so it goes over the irregular surface. With care the edge that will meet the door jam will remain full thickness, keeping that joint looking as clean as allowable, provided the conditions you are working with.

I would use PVC over the choice of wood since it is impervious to rot.