irobinovitch just corrected me that the library that provides Markdown support for Sphinx supports the features [1] that I care about; have to dig into the details but if the feature parity is very good and you strongly prefer Markdown over reST then I would say... go for the Markdown!

[1] e.g. https://myst-parser.readthedocs.io/en/v0.13.7/using/syntax.h...