I’m using a Boox Leaf 2 myself, it’s basically just an android tablet with an eink screen. I can load pretty much any eBook format, and you can put regular android apps from play store (or any other app store) on it as well if you want. And they have a microSD card slot available from the outside to expand storage.
All the hacks and mods people do to their kobo are not needed, because it’s supported by default on my Leaf 2.
IMO battery life is absolutely excellent, I brought it with me on a 2 week holiday last year, read for 30-90min almost every night and didn’t need to charge it during the trip or worry about battery life.