Rather than adding each part of the asset catalog individually, see if adding it as a whole directory works instead:
target_sources(${TARGETNAME} PRIVATE Assets.xcassets)
set_source_files_properties(Assets.xcassets PROPERTIES
MACOSX_PACKAGE_LOCATION Resources
)
Adding directories rather than files is not officially supported by CMake as far as I’m aware, but doing it this way should allow Xcode to see the asset catalog and compile it for you. I don’t recall if setting the source file properties is required or not, but give it a go and see.