OK, some more positive news… I’ve just had a working setup on Big Sur where I made only the following tweaks:
https://github.com/psychopy/psychopy/pull/3309/commits/f0d3533e19b5f03d4b7a93e79ecd87b5b392750d
That was based on Python 3.8 installed from python.org so we might yet have trouble when we port that into the Standalone app bundle (which so far has been built on Py3.6) but fingers crossed it means we should be able to get something working soon!