I somehow don’t think the STLink is suitable for debugging AVR architecture, but I won’t go there…
Debugging for AVR devices is something that would be dearly loved in PlatformIO, but I think Ivan has hit a roadblock in being able to actually implement that, due to Microchips use of a proprietary debug software layer. This was the last update I remember seeing from Oct last year:
I suspect the only ‘option’ at present is Atmel Studio and a compatible debugger, unless there is some other debugging framework available.