For iOS specifically, it currently fails because we don't force set the CMAKE_OSX_SYSROOT and CMAKE_OSX_ARCHITECTURES cache variables (but only regularly set them) in qtbase/cmake/QtAutoDetect.cmake,

There is a project() call in the top level qt5.git/CMakeLists.txt file, it means those variables get auto-populated by CMake.

Most likely we need to conditionally include QtAutoDetect from qt5.git/CMakeLists.txt before the project() call just like we do it in qtbase. Overriding them later (in qtbase) is most likely not safe.

One can temporarily bypass the issue by force-setting the values.

But then 2 more general issues come up.

We set QT_NO_CREATE_TARGETS to TRUE in the super build, which means that qt_add_tool() calls fail due to not finding the tool targets.

If one bypasses that temporarily by commenting out the line, the next issue is that the find_package(Qt6) call in qtsvg fails to find Qt6, presumably because of FIND_ROOT_PATH / sysroot issues. Setting FIND_ROOT_PATH_MODE_PACKAGE to BOTH is incorrect, cause then the host Qt6 is found.
Most likely we need to prepend the qt6 cross-compiling super build folder to CMAKE_FIND_ROOT_PATH.