The Doxygen "Modules" page is not very illuminative, as different parts
of the API are mixed together and not all parts are included.

Try to address this:

Put all parts of the public API to some Doxygen group, usually one group
per header file. Use short, systematic names.

Make these groups sub-groups of a few top-level groups, roughly
corresponding to the different logical parts of the API (core, impl,
stream, filter, spa, utilities).