There's little precise data/images available for how that fan hub works.
Basing to manual its controlled by PWM signal.
From images it certainly can't have linear regulator, because that would need sizable heatsinks.
So not sure if it just dirtily PWM chops voltage fed to fans, or uses proper step down switcher with filtered DC output.
(PWM produces 6V square wave AC with DC offset)
Anyway three pin connectors tell it can't pass any PWM control signal to fans.
So you certainly won't win anything by buying PWM fans for it.
And you're right in thinking that it doesn't increase fan speed, no matter how hot other parts are running, if CPU isn't running hot.
Because its meant to take its control from motherboards CPU fan header.
Which is controlled by motherboard basing on only CPU temperature.
(though case temperature will have at least some effect to CPU temperature)
And depending on motherboard you might not have much/any real control to control curve of that output.
That applies to all fan outputs of motherboard.
Some expensive motherboards have more controls and Windows software for that.
But they're still limited by decisions of motherboard maker... And vulnerable to usual problems from Windows:
https://forums.overclockers.co.uk/threads/case-fans-running-at-100.18828621/
So if you want guaranteed control over fan control curves and what's used for controlling them, getting Aquaero LT would be best option.
https://www.overclockers.co.uk/search?sSearch=aquaero+lt
It runs on its own microprocessor so it doesn't care, if whole Windows or hardware crashes.
You can use temperatures from physical sensors connected to it, or readings from monitoring software as basis for controlling fans.
Or just combine software temperature reading with physical sensor (attached to base of CPU heatsink etc) to have back up in case Windows crashes.
And obviously you can just disregard available fan controls when looking new motherboard.
Old Aquaero 5 has only one channel supporting PWM control signal and always uses voltage control for other channels.
Newer Aquaero 6 supports also PWM control for all four channels, besides having modern step down regulators for higher output power.
Though with quietness as goal you won't be running power hogging high speed turbines.
https://forum.aquacomputer.de/weitere-foren/english-forum/103573-new-aquaero-6/