Yep your wheels are too aggressive offset. +45 is about the lowest you can go with a 225/40/18 on the rear without catching the back of the guard. Guard rolling may help a little, but from memory it's the screw bit at the back/top of the guard where the bumper is attached that catches and you can't roll that bit. You may be able to cut or "adjust" to fit.
I had +42 and it rubbed with 225/40/18.
Changing the shock won't solve the issue, only a skinnier tyre or higher offset wheels. You could go to a 215/40/18 all round, as 215/45/18 might still rub.