K2 - tactile switch directly to ground.
[edit a few days later]
While digging thru the SAM D21 spec sheet for something else, I came across this:
[reference: page 1022 of SAM D21G Datasheet]
I don’t see any reset circuitry on the Xiao spec sheet, so it looks like I will need to add a resistor and cap to rev 2 of my PCB. 