Starting over with Qt5 and verifying that all patches are correctly applied, I encountered this error on Monterey, currently no fix. At least I'm not sure what to do so can wait until someone figures it out.
https://bugreports.qt.io/browse/QTBUG-98826
Qt patches applied are the ones for qtwebengine mentioned above, the relevant ones in the Sigil repo, and these from that repo on building 5.15.x as Universal:
https://github.com/crystalidea/qt-build-tools
As for the error in the previous post, I found this patch:
https://github.com/andfoy/qt-recipe/...t16-conv.patch