I’ve finally found the solution - it was previously documented on platformio.org here: Problems starting debug session with jlink on feather M0 - #8 by manuelbl and /10
The bootloader must be unprotected and overwritten by making the modifications indicated in adafruit_feather_m0.json file (renamed to _no_bootloader) : change “ldscript”: “flash_without_bootloader.ld” and change “offset_address”: “0x0000”.
Also change platformio.ini board = adafruit_feather_m0_no_bootloader.
Once I made these changes the USB enumeration is successful upon starting the debug session and clicking “Continue”.
Thanks to prototypicalpro!