I did manage to get the board not to go to maskrom mode by following the steps found in https://wiki.banana-pi.org/Getting_Started_with_BPI-M5#Android
I do not recall where I did get the “aml_upgrade_package.img”, but I did download it from https://download.ods.ninja/Android/firmware/radxa0/aml_upgrade_package.img, after checking my browsing history
After a successful burn, I stopped then close the application.
Now your board should not go to maskroom mode as soon as you plug USB-C into the board, back to normal operation.
Follow the step found in https://wiki.radxa.com/Zero/install/eMMC to get your board to act as USB flash
Note:
I end up in maskrom mode stuck at the start after successfully burning one of the two images of Manjaro (minimal, xfce) to the eMMC. The image could be found at https://github.com/manjaro-arm/radxa-zero-images/releases/tag/20210906.
Hopefully, this would help you out.