I didn't know I could do this
Turns out lian li fan speeds are individually adjustable on daisy-chain. (I stopped the middle bottom fan to show it on camera.) L-connect not giving you an option doesn't mean the hardware isn't capable. Maybe they accounted for fan beat frequency?
Noctua did a whole research about fan beat frequency. If your fan groups spin at very close but not the same speed (happens very often), you will hear alternating humming/whoosing sounds because your fan noise frequencies will oscillate individually, sometimes overlap, but never match each other. Setting fan speeds at least 50+-rpm apart makes your fans emit a fixed fan noise, which fixed the haunted fan noise coming from my AIO fans above 880rpm. Cheers!
(Fans: Lian-Li sl-inf flex wireless. Software: hellonexus rgb control)