Also new here, just joining to reply.
Ran first into your ESP32 epub reader repo and then eventually into this via lots of random google wandles. I have an m5paper and have been looking into how I'd get the epaper working with esp-idf. My specific use case is a bit more broad than an epub reader but I'd love to help you with you with it (at the very least with testing / debugging time, unsure about anything else just now).
This morning after many weeks of search I randomly came across: https://github.com/lovyan03/LovyanGFX
To be honest I'd say based on my reading into m5paper that the battery life will be the main issue. The controller used for the epaper is relatively well understood, together with it's limitations. But their seems to be some issues with the battery / power management. Unsure if your specific use case is untethered or if you're still working on the (admirable) goal of creating an open epub reader with extended input device support / accessability.
Hope this is helpful to you, really looking forward to any news from your end, I'm not sure I can be counted as part of the m5stack community but at the very least I'm certainly a potential future user ;)