Followup in the spirit of documenting it for someone else: If you modify the fonts in qt5ct.conf, removing the last option, for some reason it does exactly what I want: looking at notepadqq, I get a bold menu, but non-bold body text.
[Fonts]
fixed="Go Mono,11,-1,5,50,1,0,0,0,0,0"
general="Helvetica,11,-1,5,75,0,0,0,0,0"
It seems like removing the last paramater treats the specification as less prescriptive-- places in the UI that call for bold get it, and non-bold gets it. This is evident in the Double Commander Qt package, where some parts of the UI are bold and others aren't.