It all sounds like two different systems would work with some tradeoffs for each. Either method I would carry max of 30 pounds even if there is space for more items

1. Two back panniers, two back panniers, handlebar bag - I would to distribute weight between the front and back panniers. From everyone's post it appears that bike handling would be improved with this method. The one downside would be adding a bit of extra weight compared to the 2nd method. The front rack would have to stay (2lbs) and two front panniers are about 2lbs total
2. Two back panniers, some type of bag that could be mounted on the top of the back rack, and a handlebar bag. This would be about 3 pounds less weight than method 1 but at the cost of worse bike handling

I think the best thing to do is load my 520 with my two back panniers with 25-30 pounds and go out and do some riding. If the handling is poor it might be time to purchase two front panniers