Update 2
Some investigation showed that I probably have a hardware or even mechanical problem with my Librem 14’s keyboard. Intensive typing could cause the system to crash, regardless of BIOS/Firmware versions, on both NixOS (my main OS) and PureOS from LiveUSB. When the crash happens, I see the said error messages from Wifi, as well as messages from HDD controller and from other devices. I am in contact with tech support regarding this case.