The controller that came with my Simsonn Sim Racing Pedals is a cut down version of freejoy.
I installed a larger 64kb chip on the controller but the pcb leaves Boot1 floating.
This causes it to intermittently boot into either normal or flasher mode so I disabled the onboard flash mode.
See Releases page for binary
I flashed with
st-flash erase
st-flash write SIMSONN_FreeJoy_v1_7_1b3.bin 0x08000000
For some reason the app part of the program doesn't compile properly on my system so I just used the binary compiled by FreeJoy