For what it's worth, I mitigated the issue by moving the external HDD to a USB 2.0 port and limiting my download speed to 10 MB/s. It seems the best way to prevent the issue it to not do a lot of I/O, so not ideal at all but at least I can use the system again. I'm still having trouble unrar-ing a large 30 GB files though.
I am not even sure it is limited to USB usage. I setup a second rpi4 as an nfs, and ran a docker torrent client on Libreelec accessing its share over samba, and I still got the error. This is backed up by this comment on github where a user is able to reliably trigger the error with a script that employs a using a virtual filesystem. Oh, I see that you are already participating in that issue thread.
My rpi4 running raspbian with the 64-bit kernel handles these docker containers without breaking a sweat. I would very much like to address this somehow, but at this point, I am not sure where to start. It seems that this is unlikely to be fixed in the 32-bit kernel upstream when arm_64bit=1 works in most use cases.