Nice ideas. Curious why you prefer the Kobo Glo HD to the newer Clara HD? It has the adjustable frontlight color which the Glo HD lacks - one of the 'newer' tech features of eReaders.
I like your idea of the waterproof coating. Tell me, do you think there is any issue with the eInk screen getting wet - it does not have a conformal coating. I would also suggest applying 100% silicone caulk to any connectors (as Kobo factory does) to improve waterproofing the device when using a coating.
If you're looking for some battery options, I found these two candidates. They are both 4mm thick, which is 1.2mm thicker than the stock battery, but they should fit within the dimensions of the stock battery in length and width.
https://www.aliexpress.com/item/3-7V...35b63c00Z9e7eM
https://www.aliexpress.com/item/Newe...35b63c00Z9e7eM