Noted, thank you. It's odd that it's functioning exactly like I want it to, I guess ignorance is bliss.
What I described in post #7 did work, I tested and verified - linear regression isn't rocket science. I like to tinker and wanted to see if the PWM would work since I read that many Shapeoko owners over in the Carbide3D forums use the PWM from the Shapeoko controller (7.8kHz 0-5V PWM) with the Huanyang HY model. I learned along the way that the VFD can indeed accept a PWM on the 0-5V input, and that it works best on frequencies above 1kHz. My hope is this post is insightful to someone in the future.
PD005, PD011, PD072, and PD073 can be related. See the attached chart I created to illustrate. For instance, even if PD072 is set greater than PD005, the output frequency is clipped to PD005.