I dont recall ever hearing of anyone using the FSK functionality of the SX1278 in that way, so you may struggle to find anyone that knows enough to help.
We got here a old ST62T65 uC that getting data directly from a radio-module.
The originally radio-module is not longer avaible. So we have to find or design a radio-module/pcb-module that only pushing the incoming radio data to a digital I/O. The FIFO/Packet-Handling is realized in the ST-uC.