Radiator size is not going to lower you GPU temp any more than it is now.
GPU water block efficiency, water temp in loop and ambient temp in room are going to have a bigger effect on lowering GPU temp.
The water temp in the loop will never get below the ambient temp in the room, no matter how big or how many radiators you have.
More radiator surface only means the loop would be able to move more heat from the water\radiator to the surrounding air before becoming saturated and unable to keep up with removing heat from the water.
FPI density of fins on rad and type of fan used are very important for efficiency of the radiator(s).
A good 240mm radiator should be more than sufficient for a RTX 2080Ti unless it is an EVGA KingPin model.........
As said before, 45c is pretty good for a RTX 2080Ti under load on water.