Yes.. you would need to delete the bootmenu.scr in MyDocs (the real one, in the eMMC) so that it can proceed to the "next" one, i.e. your SD card.

So much for the theory

Yesterday I could verify that "0xFFFF -m u-boot-2020.12-pali.bin -l" followed by "0xFFFF -b" could boot me into Leste (but using the bootmenu.scr found in the eMMC).

Note the u-boot-2020.12-pali and not the (older) u-boot-2013.04-2.bin.

That means that in theory I could now wipe my MyDocs partition and use the newer u-boot to boot Leste.

Maybe I'll do it this weekend (I hate it when you just want to try "one thing quickly" and hours later you're still feeling miserable for the snafu you just created .

I'll certainly report back if/when I've done it