It took me longer to implement than expected, but was very rewarding. The debugger is not done in 6502, but in the Teensy-level. Much easier and quicker to get at all the stuff, and also much easier to format responses. Now I can set a break point, single step, look at memory, set any memory/registers, do NMI/IRQ and run again.
My first session dealt with single-stepping startup code from reset, and how IRQ and NMI was bouncing around in my scheduler. Turned out I had counted stack positions wrong in my threads. It was indeed a quick session to realise what was wrong :D