TeXmacs will probably meet my needs then ... not as much sizzle and flash as reveal.js, but better WYSIWYG support for technical content.

For anyone still looking for an open source editor:

https://docs.deckdeckgo.com/

https://pencil.evolus.vn/

http://thomas.nadev.net/notes/2017-inkscape.html

https://github.com/rainwoodman/inkslides

https://sozi.baierouge.fr/