Our older (1983) E350 chassis seemed about the same way. When we got it, the thing just barely stopped. Front brakes were rusted together, so replaced shoes, calipers, rotors, hoses. Helped a lot, but rear didn't seem to be doing its share. Removed drums and found new shoes, drums that had been turned and lathe marks still on them, and STUCK wheel cylinders. Replaced all... Shoes, Hardware Kit, Cylinders. Probably should have had Shoes "arched" to match drum diameter but like you did not. Braking was better, still soft pedal. Replaced master cylinder and booster, same... I believe I also swapped the proportioning valve with a used one. Disappointing braking system. It did not have RABS.
