Hi! try to re-format your SD card to be portable storage (not internal storage) and report back if this fixes your issue.
Same issue with me. Memory card as internal storage: failed update.
Then formatted as external, tried OTA again and: finally it worked.
But now i have to bring back music and fotos to the phone.
I suggest for future updates to ignore the system-default writing memory and hardcode the use of internal memory.
Greetings and thank you for still updating the device!