GFO is really easy with a two little fishes reactor and a small ball valve to control flow. Bio pellet reactors are a bit more and a little more complicated. You could avoid the replumbing and just add a small pump to the two little fishes reactor in your sump and then you could remove easy if you don't really think it's doing enough.
Plenty of people run high phosphates and get away with it just fine. A ro/di unit imo is a must. You can get them used on here for less than a $100 and it will pay for itself in time and money.