@dredlok706: I still have the issue, but your answer helped me greatly in diagnosing the problem

I have now also tried u-boot-2020.12-pali.bin.
But without surprise it shows similar behaviour (except that after failing to boot it somehow receives input which looks like AT-commands for the modem)

So no solution here, but thanks for the suggestions!

I guess my only option would be to dive into the source of U-Boot and compile a version for myself which prefers booting from SD, or even improve it to handle a damaged eMMC well by searching for the scr file on the SD.

Or maybe I could bribe pali to do it