Please start the qt6 build from Terminal.app with /Applications/Anki.app/Contents/MacOS/anki - maybe the text it outputs may give a hint as to the issue.
You could also try adjusting the video settings to see if it makes a difference, eg
QTWEBENGINE_CHROMIUM_FLAGS="–ignore-gpu-blocklist"
flags listed at List of Chromium Command Line Switches « Peter Beverloo
QSG_INFO=1
QSG_RHI_PREFER_SOFTWARE_RENDERER=1