I’ll just say I did get it working last night, it works great, and I redid the calculations for transistor loading, PCB trace width ampacity, and cable ampacity. There’s a ton of headroom and like @Hauke said you could put about 10 fans on this circuit and be fine. Which is convenient because I’m thinking about adding a second one.
@Franky, thanks for the numbers on the hot end fan, I’m referring to the part cooling fan though, the tiny 2510. I haven’t experienced any issues with the hot end not having sufficient cooling.
The control circuit is going to be a pin output from the MCU directly driving the transistor. The transistor output goes directly to the fan ground and switches ground.
There is no flyback circuit.
To limit how off topic this post got, I moved my measurements into here: