Thanks for the details. The fan speeds seem OK to me, but I'll comment on two items to check. By the way, I assume that the rad fans for the AIO system are set up to push air from inside your case out through the rad (at the case top), so they act to exhaust air out of the case. Right?
The two rad fans are turning fairly fast for a no-load situation of simply idling. Now, that MAY link to how iCUE works. Unlike some AIO systems, iCUE will alter the speed of the pump according to the CPU temperature. It's a kind of setting-the-range system. It runs the pump more slowly at low temperatures, but at higher workloads and temperatures it speeds up the pump so the rad fans do not have to reach their full speed in order to keep up with the heat demand. So look around in iCUE for details of how that PUMP speed is set. It MAY be set manually by you, or it may have its own automatic adjustment system.
The rear fan is turning faster that the front ones. I'm not sure why, but it certainly appears that is being managed by iCUE. MAYBE it is that way because the front fans are different models from the rear one (I bet rear has no lights, for one thing), and that rear one is designed to operate at higher speeds. It is likely that iCUE is simply sending to all four of those fans the SAME signals. But those signals are NOT to set each fan's speed. They are only set to ensure the right amount of heat removal via air flow, and the control system really does NOT care what the fan speeds are. So, given identical signals, fans of different design will normally operate at different speeds.