I basically have the exact setup that you do. I am adding a 4th reversible pump. I'm using the center drain plug as the thru hull, so no drilling. I am using the added pump to fill the bags under the rear seats. I'm going to use one pump and one wye fitting to fill and drain both bags at the same time. I will vent into exsisting vent lines.
If I want to list to one side or the other, I can let ballast out of rear locker bags.
Total parts via WakeMaker using coupon code is $550.