This is nearly the worst possible heat transfer configuration for this system.
First you should be running all of the cooled components in parallel and not in series. Go buy yourself a bunch of tee’s for the coolant tubes and plumb it like this
Reservoir to pump.
Pump line then has multiple tees off the main line, with a single hose going to each coolant block. This way all components get the coldest water possible.
All the outlets from the coolant blacks tee back together to the drain hose. This line goes outside the case.
The drain line then tees into the big radiator AND the small radiator that you have moved OUTSIDE the case. This way both radiators have the hottest water flowing into them, the lowest water velocity and therefore the most cooling time.
The two radiator drains then tee back together, and flow back into the reservoir.