Turns out the firmware not running is probably due to ezWS2812 debug counter not running again.
Firmware runs fine when this library is not used. I’ll use a timer instead.
USB enumeration issue goes away when not using Serial.print… This is a common issue I’ve encountered before when trying to output data on a CDC port that is not connected breaks the device’s USB stack/hw until power is cycled. The SAMD11 firmware will need updating to block UART access when VUSB is not present. A workaround would be to detect VUSB somehow and not output any serial data, but can still end in a race condition.