$ cat rpi-kernel-upgrades-response.txt
@popcornmix thanks I didn't experience any obvious improvement after upgrading to da59cb1161dc7c75727ec5c7636f632c52170961 (kernel v5.10.3) and unfortunately the hard lock problem persisted, I have rolled back to dea7234943c604462e476a8afc13c587418e8709 for now.

This seems a common precursor to the lock from syslog
Dec 28 01:57:14 twofar pulseaudio[851]: E: [alsa-sink-bcm2835 Headphones] alsa-sink.c: ALSA woke us up to write new data to the device, but there was actually nothing to write.
Dec 28 01:57:14 twofar pulseaudio[851]: E: [alsa-sink-bcm2835 Headphones] alsa-sink.c: Most likely this is a bug in the ALSA driver 'snd_bcm2835'. Please report this issue to the ALSA developers.
Dec 28 01:57:14 twofar pulseaudio[851]: E: [alsa-sink-bcm2835 Headphones] alsa-sink.c: We were woken up with POLLOUT set -- however a subsequent snd_pcm_avail() returned 0 or another value < min_avail.