As an update for anyone who comes across this in the future, the Hepa Test method did the trick.
The issue with the method I was using was a misinterpretation of one of the input parameters for the set fan speed function. In the HxFan library, the particular parameter (forgetting the exact name) is described as “rate in which the device will ramp to its max speed”, but in reality, it is actually looking for an input of “what % of max speed do you want the fan to run at”.
Thinking it was rate, an input of 250 that I was using (250 rpm/sec ramp speed) is reasonable, but a fan speed of 250% of max speed, which it was actually interpreting, is not!