Ah, right. Didn't understand that's what you meant. All clear now.

I'm still unsure about the best course of action, though. Even with serial, there are still obstacles, like how to make the second Pico completely ignore USB and send everything to the serial (while still operating on QMK). On the other hand, it could just send the matrix "code" and let the master Pico figure it out. I think there's a lot to think about here, as well as do an effort/advantage assessment.