You can get information using ssh, but I couldn’t find anything to set fan speed.
Your idea of setting the speed to 80% would work if your NAS CPU never reached 100% or if your room is always cold. But imagine if you need more cooling but your fan is set to 80%, you could possibly damage your NAS due to overheating.
Maybe my fan is high because my NAS is sitting close to my heater. If your fan speed is that low and the temperature under control, it means your room is cool enough.
Don’t focus on fan speed, but on temperature.
root@MyCloudEX2Ultra ~ # fan_control -g 0
Current temperature is 57
root@MyCloudEX2Ultra ~ # fan_control -g 4
fan rpm = 3448
root@MyCloudEX2Ultra ~ #