Some time ago, I reversed the protocol used by KBT Pure Pro for flashing its firmware in a hope of being able to flash a custom firmware (or do some BadUSB demonstration). This script is able to upload a firmware binary onto the keyboard.
The binary is obfuscated (already solved on SE) However, the firmware has a 32-bit checksum (at the end of the image),
which does not seem to be any of the commonly used ones (there is this) (EDIT: Actually turned out to be a standard one, see edit below).
If the uploaded firmware checksum does not match, the keyboard stays in the bootloader.