I’m betting the brushes and commutator surfaces are OK and the problem lies with the commutator shorting out to the lower end plate. The weight of the armature wears out the thrust washer(s) allowing the ends of the commutator segments to make contact with the end plate. looking at your photo I can see the large circular area on the end plate probably caused by the segments dragging and the marks sure look to be the same diameter as the commutator. The way you describe the starter being sluggish to is also a sign of an internal short.
Did you find a thrust washer or two in there when you took it apart? If you clean up the plate and lay it over the end of the armature you’ll see how close together the parts are. In these cases I make new washers out of Acetal (Delrin) which is non conductive and also acts as a bearing surface.
