Why are my brake pads completely eroding after one week
It sounds like you are buying unusually quickly wearing pads. 900 miles is a short distance from a set of brake pads, but 5-6 sets over 400 miles is really excessive.

Personally, I have had very good mileage and all weather performance with Kool Stop Salmon pads. The original pads from Shimano are not particularly good, but not the worst either. There are some other brands that have good reputation, but I haven't tried all of them.