PlatformIO with STM32F303RET6

I have a homemade PCB with STM32F303RET6. The PCB is working, I can program it using CubeIDE + StLink V2.
I want to program it using PlatformIO, but there is no board for STM32F303RET6 (generic). I tried using the board for NUCLEO-F303RE (nucleo_f303re), but there are problems during flashing: I need to press reset at the right moment, otherwise it doesn't flash. But even after flashing, the program doesn't work (just turning the light on and off).

I am looking for someone who understands PlatformIO and can help flash the program onto the PCB preferably without pressing reset and to ensure the program works (a test program that writes via UART and turns the light on/off is quite sufficient). The PCB is working, I can program it without any problems in CubeIDE.