I wanted to follow up with two possibly relevant things I’ve learned, not for @NotTheMama who has a working solution, but for whoever next finds this in a search… 
The first is the discovery that my new enclosed printer prints passable threads in ABS without post-processing with a tap, and without any modification to the thread form from the “ideal” thread form in the model. I think that ABS shrinks more than PLA, so that was a surprise to me. That’s an argument for it being reasonable to expect a well-tuned printer profile to print fairly accurate internal threads without modification to the model.
The second is that at least in the current 0.20.1 FreeCAD I’m running now, in the current Fasteners Workbench, there are preferences for modeling for 3d printing that allow you to set scale factors that are then used for modeling thread forms specifically for 3d printing.
You basically subtract the metric or imperial threaded rod from a hole, and if you set it to be 3D optimized, then it uses the adjustment from the configuration.