Eh, I am controls Captain, I think of fixes on the control side. I also like my wires going into the same place. Although I don’t think grounding to the PDB will work unless you put it on a socket that already has a load on it, which I don’t think is allowed (one wire per wago).
The black terminals on the PD are always connected to the negative terminal of the battery even if there is no breaker in place.
No need to cut up a Pwm cable, just wire your load like this.
Or you could use a linear regulator with an enable pin. That would eliminate the need for a spike or separate FET.