qtwebengine 6.3.x update: Might have finally found a way to work around too long filenames, waiting now to see what happens:
Qt 6.4.2 update: qtbase, qtmultimedia, qtshadertools, qtposition, qtsvg, qttools are built (not tested), building qtdeclarative now, then qtwebsockets and qtwebchannel. I won't even start thinking about qtwebengine until I have the webengine from 6.3.x built and in a similar state to the 6.2.4 webengine...