Unfortunately, blackmagic currently doesn't support MK20 chip used by the Teensy.

There's an old PR where someone tried to contribute to blackmagic by adding support for MK20, but sadly it never got in. That can be used as a base with the hope of pushing it upwards if it succeeds.

From here on, I'm in a new area for me. Even though I've used several debuggers in development, my knowledge of what makes them tick and GDB console is pretty much none-existent. This is going to be fun!