That error is also something that i always had trying to update with sideload.EEA and still seen as EEA.
When im trying to update Via adb sideload im getting error:
E: Failed to unmount /mnt/product/nt_log
Maybe because of that i can update?
If you can boot a patched boot img maybe we can find the reason for error 20, but it can be complex, are you in?
It can help a lot of people, using a terminal is required, but it shouldn't be too hard.
Termux or bash from adb debugging will be used.