In case it helps ...
My own Sigil repo:
https://github.com/kevinhendricks/Sigil
has been updated to pull in all of the Qt6.2.2 and Python 3.9 fixes along with all of the required Qt 6.2.2 patches and build instructions.
So if you clone master on that site or download a zip, you should get Qt6.2.2. patches and build instructions in Sigil/docs/. There are also new build instructions to build Python 3.9.9 with its own embedded TclTk and PySide6.
Once those patches are applied Qt 6.2.2 should build out of the box on Apple's M1 (arm64). Python 3.9.9 should also build out of the box on Apple's M1 (arm64) and of course we have updated the Sigil master to work with both Qt6.2.2 and Python 3.9.
No universal builds should be needed and most if not all of the prerequisites can be be either built from source or installed via HomeBrew.
See:
https://github.com/kevinhendricks/Si..._on_MacOSX.txt
and
https://github.com/kevinhendricks/Si..._on_MacOSX.txt
and the qt622_*.patch files and python_3.9.9_fixes.patch in
https://github.com/kevinhendricks/Si...ee/master/docs
I have built this all from source on macOSX 10.15.6 with XCode 12.4 on x86_64 and tested it and it seems to work well.
All of these changes will make it into Sigil-Ebook Sigil master sometime after the holidays.
Hope this helps make Apple M1 arm64 support a bit easier.
KevinH