I am not sure if you are now using the bootloader of if you are still loading via usb to serial
I you upload the bootloader, you should then be able to connect the PC directly to the USB on the STM32,
However it looks like the board has reset hardware like the Maple mini, so the bootloader you need to use is probably the maple mini version
But i cant read the circuit as it is too blurred
can you tell us which GPIO pin is connected to the 2 transistors which are in the top left of the diagram?
Perhaps its PB1??
But the Maple Mini uses PB9.
I think someone added another Maple mini variant bootloader with using a different pin ( to do the USB reset), but I cant remember which pin that was.
So I may need to add a new bootloader variant to support this board if it uses PB1