@zapta I'm running all Duet hardware. Duet 2 Wifi, and a smart effector. I ditched the stock electronics before I assembled the printer. The printer frame, PSU, and heated bed is about all that was worth keeping from the original kit.
I've got a 50mm axial fan, which seems to work somewhat with PWM and isn't annoyingly loud. It's mostly just the blowers that area a headache.
I've been planning on getting a second smart effector for use with a Volcano nozzle once Spool3d starts carrying the v3, so having to get 6 fans with 24v and PWM is a bit problematic if I can't be sure they'll work. It seems like the buck convertor might be the best option, which is frankly kind of surprising.
A Berd-Air also looks like it might be a good option. It should be fairly easy to switch between smart effectors and they appears to have a 24V PWM model. It sounds like it might be a little on the noisy end without an enclosure though. Anyone have any opinions on those? The shipping isn't cheap on them, but they look like they provide decent adjustability and low effector mass.