Yes, surely it's an easier way, but that's just a starting point for something more complicated where I won't flash a clean image (but a customized one that should be different from the OS used for flashing) and, last but not least, I'm interested in knowing where the problem is
UPDATE: Ok, I reflashed it and now it works
So flashing through dd seems fine
From serial monitor I see that sometime it hangs on
U-Boot SPL 2018.11-armbian (Feb 08 2019 - 11:35:32 +0100)
DRAM: 512 MiB
Trying to boot from MMC2
that's what was happening before, but it's another kind of problem (looks like u-boot for h5 has a few issues)