Are you able to boot from USB on the Radxa Pi Zero?

[RadxaYuntian]

Introduce the Radxa Zero

You should be able to use eMMC to microSD adapter like this, although I don’t have one right now to test for you. Clear bootloader in eMMC , then flash your system image to a thumb drive and plugged in without any microSD card inserted. You should be able to boot into USB. If you are still having issues you might need to manually fix your boot order with a serial cable.