Jack,
Can you solder?
At very least I would advice you to spend $10 on STLink V2 and have some patience.
I just confirm that 4-pin header is working STLink interface. I’m trying to do the dump and copy. (to copy working JL1 to Blue Pill). Will report if sucsesful. If so - we will have safe recovery option for all.
You also need to solder 4-pin header into the board and to have 4pin jumper cable female to female.
I will provide instructions when I have all files ready.
Y’all: does anybody able to successfully flash their JL1 to GRBL?