Using the downloaded source for Qt 6.2.3, and using QtCreator, compiling the iOS source for QtMqtt module succeeds.  However, the process writes the header file paths incorrectly in the

/Users/login_name/Qt/6.2.3/iOS/QtMqtt/include/QtMqtt 

Including:

qmqttclient.h

and all of the other lower case "q" header files.

The path example in qmqttclient.h i

#include "../../../qtmqtt/src/mqtt/qmqttclient.h"

but it should be:
{{}}

#include "../../../Src/qtmqtt/src/mqtt/qmqttclient.h"

Once these changes are made in the offending files, the library seems to be fully functional.