Well, I did the manual update, going from fw 5.12.3 to 5.13.2, and it seems like the battery performance has improved. I've just read for over an hour, and lost only 2%. Before, I was losing over 5%. This is the performance described as good by Nathan at the Ebook Reader.com:
https://blog.the-ebook-reader.com/20...the-same-time/.
Hopefully, the sudden 20-30% drain overnight will have been stopped as well but will wait to see if this has been dealt with.