Pardo [...] What I tried so far:
Full factory reset from recovery (and rescue) modes.
This is concerning, as follows. If you have wiped the user data partition and (discussed below) also installed a new system partition, and the device's hardware is ok, I think it should boot.
Pardo The problem comes in when the GrapheneOS logo appears. It has the shadow moving from left to right for a random time (can vary from maybe 5 to 30 seconds) after which it freezes (no shadow moving, just the white logo) and then reboots itself.
I am nervous about that range of variation. If something is going to go wrong and panic/crash the OS at that point, it sort-of should be deterministic. If it's varying all over the map that might be consistent with a storage failure.
Pardo 1st update (stable): Installed succesfully. On reboot the boot slot was not changed (slot b). And, when I entered recovery to do the next sideload with adb, it warned me that an update was in progress and that if I continued it would be lost. So I restarted and entered recovery again, in which now I could do adb sideload without warning. Does this convey any info?
I'm afraid I don't like that either. I think that if the sideload completed, the update to the system partition should be done, not in-progress. That sounds like maybe some things were written to storage but weren't there after the reboot.
Going back to the start of this... is it clear that the problem started after the battery was allowed to drain to zero, or is it possibly the other way around? Might the system have locked up and sat there, or maybe crashed into a boot loop, thus running the battery down to zero?