(Dropping this here in the hopes that it'll help someone else with a similar setup, because I had to go digging through a ton of old discord conversations to finalize some of this, and a search engine might actually pick this up.)
I have a Klipper (well, Kalico)-based 3d printer that I'm putting together, and the hotend has a couple of those honkin' 12V Delta 3628 4-wire fans hanging off of it for part cooling (an Annex K3 with SLM toolhead in my case, but it's a similar story for Takeoff, Calamity, Trinity, and other 3628-based toolheads). I wanted both proper PWM control of the fans (there've been reports on the Annex discord that these fans can do Very Bad Things to your 12V rail if you just soft-PWM them in a 2-wire configuration) and a usable RPM reading in Fluidd.
Wiring specific to the K3 and my Octopus Pro: at the k3rabiner (toolhead) side, I solder-bridged the part fans to "alt" voltage, wired +/-12V on each fan to their corresponding 2-pin part fan JSTs on the board, and both pairs of PWM and tach