Just an update - shortly after I tried testing the 5.14 kernel,
the microsd card in my pi4 died. While I waited for a new card to
arrive, I also picked up a much faster wifi adapter. So while I'm
still using the brcm80211 chip for our 2.4ghz AP, I have a 5ghz
that every other device in our house except for one is using. So
despite still using a 5.10 kernel, I haven't been able to
reproduce this because the 2.4ghz wifi just isn't that busy.
Clearly the bug was being triggered by either multiple 2.4ghz wifi
devices, or a lot of 2.4ghz traffic (or both).