Thanks for all the details and discussion on this topic. What you have figured out is correct. If the device has a working u-boot then you can modify the uboot to boot from SD card or USB drive, and this could save you.
However, if you want to write an image into eMMC once the device is bricked, you could first boot Toradex Easy Installer (TEZI) from an external medium (SD/USB) and then auto-install your custom TEZI compatible BSP/Torizon OS image. This solution is shared on this post: Verdin-imx8mm - easy installer and custom image in a single place - #7 by bruno.tx?
Let me know if this helps!