For example, h1 and h2 would become slides. h3 (or a separate #Sphinx directive) would have font size reduced in presentation mode to not take attention, but people could see there is more + in the free-flowing text it provides the story that would be said (not read) when presenting - since I'd expect more people to read later than watch the presentation.

This is #minipres: https://github.com/coderefinery/sphinx-minipres (probably #SphinxRtdTheme only)