Might it be "normal" that the battery degrades, due "calendar-aging"?
https://www.zitara.com/resources/lit...ry-degradation
I've been surprised to read into that link that USB C fast charging would degrade even more the batteries.
I recall on Kobo AuraHD the battery would last much more that the Forma's, but as you did mention its capacity was bigger, and the different size of the screen, or the PCB on which is based requires more power too..