Please try a minimal Arduino sketch with USB serial first, as Marlin is huge and modifications to it might interfere with something else.
PIO knows 4 boards with the exact STM32F407VGT6 MCU already, I think you want to start from there ( black_f407vg, disco_f407vg, genericSTM32F407VGT6, diymore_f407vgt).
E.g., as platformio.ini:
[env:black_f407vg]
platform = ststm32
board = black_f407vg
framework = arduino
; uncomment to change upload protocol from stlink to dfu if needed
;upload_protocol = dfu
build_flags =
-D PIO_FRAMEWORK_ARDUINO_ENABLE_CDC
-D USBCON
-D USBD_VID=0x0483
-D USBD_PID=0x0003
-D USB_MANUFACTURER="Unknown"
-D USB_PRODUCT="\"black_f407vg\""
-D HAL_PCD_MODULE_ENABLED
src\main.cpp
#include <Arduino.h>
void setup(){
//activate USB CDC driver
SerialUSB.begin();
}
void loop() {
SerialUSB.println("Hello world");
delay(1000);
}