First question is "why don't you want to boot in Android?".
My reason was I wanted to replace the system with debian completely and start from nvme.
Experience learns one can roughly just erase the android image (can be restored if needed).
Using rkdeveloptool or (as linux user) upgrade_tool blank the emmc and rewrite the the spinor.
If you prefer a vanilla debian system try https://github.com/inindev/nanopc-t6/
Still requires you to blank the emmc and overwrite the spinor (I used the firnedlyelec sd-image for that purpose since it gives more control than using upgrade_tool).
With the friendlyelec image I suggest blanking the emmc and see if it works. If it doesn't work use the same spinor image as with armbian (the armbian installation manual uses a 2017.09 u-boot image, inindev uses a 2024.07 image). I expect the current armbian to have updated their u-boot. There are some minor nagging points with armbian making me prefer vanilla debian.