Ortlieb mounting system, you did not specify which, there are two types. I have the classic frontroller and classic backroller. Mounting system is not flimsy but it does rattle a bit. I wrapped electrical tape around the rack where the rack mounts attach and at other spots where the rack can rub to reduce the amount of paint loss on the rack.
I tour maybe a week a year so I do not have the high mileage experience of some others. I have read that after many months of use that the little clips that go into the Ortlieb mounts can fall out after the rack mounts develop a slight bend. Those clips are used for different diameter rack tubing. I also recall reading somewhere that someone on a year long tour crashed and broke the lower attachment bracket on an Ortlieb. But for my type of use, Ortliebs will last longer than my lifetime.
I have used the Ortlieb medium duffle on top of the rear rack and panniers. If I was going somewhere dry, I would give serious consideration to carrying a similar sized non-waterproof duffle for damp stuff or if I was not too concerned about peering eyes and dust I might even consider a mesh duffle.
I have also used front nylon panniers that were not waterproof that had a couple of really convenient pockets. If I was going somewhere dry, I would consider using those instead of the frontrollers because pockets can be pretty nice. Yes, you can buy Ortlieb pockets, but they would have to be a LOT cheaper before I buy them.
As the day warmed up and I started shedding layers, I strapped the excess layers on top of the frontrollers as shown in the photo. I have also strapped damp stuff on top of the front rollers. So, lack of pockets, while an issue is not quite as bad as it might otherwise sound.
I have not used Brooks or Carradice, but if you are looking at Brooks you should also consider Carradice gear.