Screen: 7.5" Monochrome eInk / ePaper Display with 800x480 Pixels (https://www.seeedstudio.com/7-5-Monochrome-ePaper-Display-wi...)
BTLE board: XIAO ePaper Display Board(nRF52840) - EN05 (https://www.seeedstudio.com/XIAO-ePaper-Display-Board-nRF528...)
Battery: 3.7V 2000mAh - (https://www.amazon.com/dp/B0FR9GH966)
Picture of it all assembled (no case yet): https://cs.joshstrange.com/zJFvGGPB
The image rendered to the screen is coming from a little service I wrote that renders a book quote it pulls from BookOrbit which syncs with KOReader (annotations, progress, etc). My plan is to set up one of these one each bookshelf/series rotating quotes I highlighted from the series.
Here are 2 ESP32-based screens I have, the one on the left is a an all-in-one [1] and the one on the right I printed the case from the TRMNL DIY kit from SeeedStudio [2]: https://cs.joshstrange.com/knk4Ns2h
Right now only the BTLE one is running OpenDisplay (the other two are semi-managed by HomeAssistant ESPHome) and all the devices are in HA for controlling them, though the ESP32's all deep-sleep for saving battery so I can can't "push" stuff to them like I can the BTLE one.
[0] https://opendisplay.org/index.html#battery
[1] https://www.seeedstudio.com/reTerminal-E1001-p-6534.html
[2] https://www.seeedstudio.com/TRMNL-7-5-Inch-OG-DIY-Kit-p-6481...
The BTLE boards have just crazy higher battery time with no need for deep sleep which is why I'm moving in that direction.
~7.5" is the size I've been buying so far, the larger e-ink screens get pricey but I can't wait for a calendar-sized one to come down in price such that I can frame it and hang it on the wall. Lastly I'd avoid the color e-ink screens unless you know you need color, won't need to refresh it often, and it's not in your eye line. Color refreshes take way longer (~30s in my experience for a 5-6 color screen) and the flashing it has to do is obnoxious. Monocolor displays take <2s normally and a single flash. I got a color screen first because "why not?" and I got color, but the refresh was too annoying so now it refreshes once a day in a place that I'm not when it refreshes so I don't have to see it.
I also recommend you check out OpenDisplay for what you will run on the device: https://opendisplay.org/index.html
[0] https://www.seeedstudio.com/reTerminal-E1001-p-6534.html
So reiterate, with a 2000mAh lipo battery I should be able to drive the board _and_ the screen for over 3 years if I only refresh it at most once every 4 hours (with "push" capabilities, I don't have to deep-sleep the board to get that battery life). At least, that's what the calculator says, I _just_ got my hardware to play with and so I can't speak from experience. Unfortunately my board is not charging the battery so I need to get a replacement which will probably take a week or two to get here.
And then you're stuck with a device that can only be communicated with when it chooses to wake up, whereas with BLE it seems you can listen for a remote OTA wake much more cheaply.