How would you run 5 independent loops off a single channel radiator?
In series, just like I used to do with 4x GPUs. Can easily dissipate 800W+ of heat on the standard 240 rad I was using back then.
We don't have 800W of heat, we have 5KW of heat - you simply cant get the flow rate required to do that in series before you get to the radiator due to the internal diameter of the tubing [and pathways in the heatsinks].
Just use a bigger pump.