I tend to carry a fair amount of weight on top of the saddle as well.

Yes, the old Carradice saddlebag has worked well for a long time. It’s interesting to note that originally it was not intended to be used with a rack and that there are versions specifically made to avoid tire rub on bikes with fairly low saddles. People often complain that the reflective decals on their bag’s flap point upwards away from traffic, well that’s because it’s designed to be visible with the bag strapped to the seat post rather than a rack.Using one with a rack is also not optimal as it moves the mass farther from the rider. The farther the load gets from the rider the greater the “tail wagging the dog” effect happens and then you want to get the load down low.