I work at an Apple certified repair center. Theres a tool called the "Customer data transfer tool" that connects to a certain port on a logic board thats hard wired directly into the SSD. On the other side is a USB C cable that is connected to the machine with a new logic board. This will work even if the rest of the logic board is dead, as the SSD circuitry is isolated.
You can read more about it here: https://9to5mac.com/2016/11/24/apple-special-cdm-tool-macboo...