I tried all the above with v 1.8.13 to no avail.
What worked for me (on macOS) was to use the doxywizard->Expert tag to fill the USE_MD_FILE_AS_MAINPAGE
setting.
It made the following changes to my Doxyfile:
USE_MDFILE_AS_MAINPAGE = ../README.md
...
INPUT = ../README.md \
../sdk/include \
../sdk/src
Note the line termination for INPUT
, I had just been using space as a separator as specified in the documentation. AFAICT this is the only change between the not-working and working version of the Doxyfile.